[RPG] If I take a feat that allows Critical Hits on undead, can I make sneak attacks too

Wizards of the Coast: No.

Analyze Unliving: Pick a monster type: Construct, Elemental, Ooze, plant, or Undead. You can do sneak attacks against a monster of that type, but they are d4 damage dice instead of d6 damage dice. (Ultimate Feats)

If you can't sneak attack, then you have one thing that you can always do and should be doing, especially at that high of level:

Use Magic Device

Use a Scroll

If you are casting a spell from a scroll, you have to decipher it first. Normally, to cast a spell from a scroll, you must have the scroll’s spell on your class spell list. Use Magic Device allows you to use a scroll as if you had a particular spell on your class spell list. The DC is equal to 20 + the caster level of the spell you are trying to cast from the scroll. In addition, casting a spell from a scroll requires a minimum score (10 + spell level) in the appropriate ability. If you don’t have a sufficient score in that ability, you must emulate the ability score with a separate Use Magic Device check (see above).

This use of the skill also applies to other spell completion magic items.

Some scrolls of Polar Ray would seriously put the hurt on fire elementals. Some scrolls of Horrid Wilting would seriously put the hurt on water elementals. The cleric, should also consider Imbue with Spell Ability to also help out those with subpar class features - and also help him out as well. The wizard, also utilizing Haste would give everyone an extra attack action and increase movement rate.

Don't think that there isn't any time to prepare. Your wizard can cast Greater Teleport (if he can't shame on him), so that you can at least go to town and buy some scrolls. At 20th level, the wizard and the cleric is simply going to outshine everyone else, especially core only parties. Let them. You can still do your part by aiding the others, distracting your enemies, giving out Potions of Healing, etc.

The primary source on Favored Enemy is either Player’s Handbook in the ranger class description, or Rules Compendium if you buy its assertion of primacy. The descriptions in the Invisibility description in Dungeon Master’s Guide, Improved Manyshot in Epic Level Handbook, or even darkness, despite also being in the Player’s Handbook, are definitely not the primary source on Favored Enemy.

Further caveats, limitations, and addenda not mentioned in the primary source description are contradictions with that description. If Favored Enemy had defined itself as precision damage, the primary source on precision damage would apply, but it didn’t, which means that neither the precision damage description nor anywhere else can define it as such.

I would be inclined, in general, to follow contradictory rules as far as they go. Darkness and Invisibility cannot define Favored Enemy as precision damage in general, but it can say Favored Enemy doesn’t work in those conditions. Here, specific-trumps-general: rather than trying to redefine what Favored Enemy is (in which primacy asserts itself), they can define a special case which acceptably contradicts the general rules. So my reading of the rules as written would be that Favored Enemy is not precision damage, but it does fail to work in cases of darkness or Invisibility, and does apply only once to Improved Manyshot.

Ultimately, however, I would mostly ignore any and all rules as written that apply any more needless limitations on Favored Enemy. Of all the iconic core class features,¹ Favored Enemy is one of the weakest, and that’s even assuming that it “just works” on any and all attacks against the designated foes (including, therefore, Improved Manyshot).

1. ^{Slow fall and wild empathy, if counted as “iconic class features,” are definitely weaker than favored enemy, and smite evil and trapfinding give it a run for its money too. That’s still a small list compared to all of the options.}