From looking at your group I would put my chips on the problem just being that the psychic warrior is optimized while the rest are not.

In a brute force manner you could do the following, but I do not recommend any of these:

1. Increase the CR of encounters. Doing so would make it more challenging for the Psychic Warrior, but would run the risk of making it too hard for the other characters.
2. Decrease the CR of encounters. Doing so would allow the other characters to be able to handle things like the psychic warrior is now, but the psychic warrior would just be stronger in comparison.
3. Nerf the psychic warrior. Doing so would make him more on par with the rest of the party, but the player would likely feel targeted and punished. In my opinion this is never the answer.
4. Buff the rest of the party. Doing so would put everyone on par with the psychic warrior, but if that player is the type of gamer who min/maxes and optimizes he will want the buffs too, and would likely feel left out.

Since none of those solutions will solve the problem in a fair way I would recommend is tuning the game to give everyone a chance to shine. A few examples:

1. For the ranger include times when an enemy needs to be tracked, or include the ranger's favored enemy in encounters more often.
2. Include enemies that are resistant to physical attacks, either through high AC or DR, that the wizard can better handle. And at higher levels include encounters with a lot of weaker enemies that can be taken care of with area spells like fireball.
3. Include more traps for the rogues to take care of, or make sure that there are enemies that can be sneak attacked. Or if the rogues are more charismatic you can include social encounters.
4. Include undead for the cleric to deal with using turning or positive energy (I assume he is a good cleric).

As a final note, your campaign is still in the very early levels. Eventually you will likely find that the Wizard and Cleric will start outshining every one else, that is just the nature of D&D 3.5. This is an idea called Class Tiers, which says that some classes are just better than others because of their raw mechanical power and versatility. This system does however rely on all characters being played at an equally optimized level, so if the one character is much more optimized than the others then this may not apply.

TL;DR Don't just try to buff or nerf the encounters or the party. Try to include situations where each character can shine, including outside of combat. The problem may just resolve itself in time as some characters become more powerful than others at higher levels.

This winds up getting away from the idea of the hulking thug rogue/barbarian; the solitary barbarian level is used to grant mobility and agility rather than strength and endurance. That said, this is a very vicious combat build with lots of dirty tricks, very capable of pouncing on opponents unawares and tearing into them in an unholy frenzy.

The Assassin

For a low-optimization, low-magic campaign, where you want to focus on stealth and skills but still be able to deal the hurt, you could do a lot worse than the core assassin prestige class. It gets a number of low-level spells, so it should not be a problem for low-magic, but if there’s little magic available, you definitely want what little you can get.

If alignment is an issue, consider the avenger: it replaces the Evil requirement with a Non-Chaotic requirement, which is often much more amenable to characters.

Spells

Spells are the reason you go this route. The assassin spell list is quite solid, for as small a list as it is, and it will likely fly well in a low-magic game.

This recommendation does assume that you have access, at the least, to Spell Compendium. The core assassin spell list leaves a lot to be desired. If you want to use the avenger, also make sure your DM is OK with adding the Spell Compendium assassin spells to the avenger list. That’s in line with the guidance in that book for non-core spellcasting classes on pg. 3, so it shouldn’t be a problem, but it’s a conversation you should have.

Psionics

The psionic assassin from Secrets of Sarlona also exists, and adds the very interesting option of using Expanded Knowledge for psionic minor creation, which is an excellent way to get poisons. That said, unless your DM allows other assassin spells to be converted into psionic counterparts, it’s not a great option. If you can, then you might as well?

Death Attack

Don’t focus too much on Death Attack; at best you can attempt it during a surprise round after spending three rounds studying your target out of combat. Attempting to hide and wait three rounds mid-combat is always a bad play.

Poison Use

Poison Use is meh; if you want to use poison, you want the Master of Poisons feat from Drow of the Underdark anyway. That said, Master of Poisons is a quite-solid feat, and Craft (poisonmaking) can get you poison at a huge discount (if you have a source of materials, ⅙ market price). Poisons provide a way to apply debilitating effects along with straight damage. See the Arsenic and Old Lace handbook for more details if you’re interested in going for poisons.

Again, if you are avoiding being Evil, you have to clarify something with the DM: poisons, according to Player’s Handbook and Dungeon Master’s Guide, are not evil, though usually illegal (and always dishonorable, which is why they are barred to paladins). Book of Exalted Deeds, however, says they are actually Evil – and then gives a definition/explanation of how and why they’re Evil that would include, for example, the natural poison of the couatl, that is, a notably Good creature. Book of Exalted Deeds is not a good book. Just make sure, if you don’t want to be Evil, that your DM agrees that Book of Exalted Deeds is stupid, and poisons aren’t any more inherently evil than swords.

Hide in Plain Sight

It’s a long time coming, but getting Hide in Plain Sight, along with taking the Darkstalker feat from Lords of Madness, will prevent a lot of the major ways to simply shut down your stealth. Highly recommended.

The Black Dog

This prestige class from Dragonmarked stacks with assassin levels for Death Attack DCs, and has a couple of excellent abilities aimed at poison. I’ve decided that poison is a good direction to take this, hence its appearance here.

The black dog requires the Mark of Hospitality feat, which in Eberron is limited to halflings. This is a matter of setting fluff, so if you aren’t playing in Eberron, the Mark of Hospitality might be available to half-orcs. On the other hand, I actually like halfling a great deal more than half-orc here. It does mean a shift from Strength to Dexterity which you may find undesirable, but half-orc is quite weak, and the halfling’s size is basically all to your benefit (average of −1 on damage rolls, but +1 attack, +4 to stealth, and +1 to AC). The existence of the strongheart halfling (Forgotten Realms Campaign Setting) really seals that deal.

Suggested Build

Strongheart Halfling Rogue 3/Barbarian 1/something 1/Black Dog 5/Assassin 10

At Rogue 2, the Penetrating Strike alternate class feature (Dungeonscape) is key, and at Barbarian 1, you really want the Lion Spiritual Totem alternate class feature (Complete Champion) to get Pounce instead of Fast Movement (you’re not a charger per se, but you do want to maintain mobility and full-attacks, and pounce is a really good way to do that).

I’d replace Rage with Ferocity, though I’d also consider Whirling Frenzy.

The something 1 I’m not sure about; barbarian 2 is OK but not great, rogue 4 is fairly mediocre...

Some options that should be no problem:

• Fighter. You want Weapon Finesse at this point, so that’s a use for your bonus feat, and the Hit-and-Run Tactics alternate class feature from Drow of the Underdark trades some armor and shield proficiency you probably won’t use for +2 to initiative and Dex-to-damage vs. flat-footed foes, which is awesome.

• Ranger. Simple: full BAB and 6+Int skills. Boring, but functional.

• Anything that gives Sneak Attack +1d6 at first level. Again, boring, but functional.

Some options that probably aren’t allowed, but could be awesome:

• Cleric. Best single-level dip in the game, bar none. Could be worth it even if you get none of its spells. Domain granted powers are awesome, and failing that you can trade them for Devotion feats (Complete Champion), many of which are excellent. Turn/Rebuke Undead is excellent for Divine feats, including the amazing Lolth’s Caress (Drow of the Underdark). While a halfling who worships Lolth is weird, with this build’s interest in poison it’s not a stretch to worship the goddess of spiders. Or if Lolth doesn’t exist in your setting, maybe you can just ignore that requirement.

• Ardent or Psion. One level is sufficient to manifest psionic minor creation, which will get you a plant-based poison that lasts an hour. Not a bad deal at all.

• Swordsage. Cloak of deception provides instant flat-footing for an enemy, once per encounter, while shadow jaunt provides great mobility. Wolf fang strike and sudden leap allow you to continue to dual-wield effectively even when you’re forced to move and cannot charge. If you later take Martial Stance, you can snag assassin’s stance for +2d6 Sneak Attack damage.

Anyway, you definitely want the Craven (Champions of Ruin), Darkstalker (Lords of Madness), Master of Poisons (Drow of the Underdark), and Weapon Finesse feats. Plus, black dog requires Mark of Hospitality (Eberron Campaign Setting).

The feats Extra Rage (Complete Warrior) and Wild Cohort could be very nice for you, if you can squeeze them in (maybe later, or if you take a Fighter level and have another feat to play with). Note that I don’t recommend Wild Cohort so you have a pet fighting alongside you: I recommend it so you have a reliable, cheap poison dispenser following you around. Unfortunately, both feats are kind of lackluster by the time they fit into the build I am proposing.

Anyway, your feats may look like this:

• Bonus racial feat: Weapon Finesse
• 1st-level feat: Master of Poisons
• 3rd-level feat: Mark of Hospitality
• 6th-level feat: Darkstalker
• 9th-level feat: Craven
• 12th-level feat: Extra Rage?
• 15th-level feat: Wild Cohort?

With this combination, your attacks hit hard, and you are hard to find. You have poisons that you can use selectively, and they are very dangerous. You have a smattering of spells that will make a huge difference in a low-magic world. Seems like a pretty solid approach to me.