How to replace a bathtub drain trip lever

The stem you speak of is actually connected to the internal cartridge. I believe it may have been over tightened when installed, or it was leaking a bit and some gorilla cranked it down. You will need to remove the bezel trim, and loosen the nut that holds the shaft and cartridge. There may be two nuts. If you loosen too much, the valve will leak. Some models require a spanner type wrench to loosen. A good close up pic without the trim would help here. If you can post the exact model number, we may be able to pull up an install detail for ya.

Unfortunately, I think you're fixin' to go for a crawl with the crawlies. (At least you have that option -- the same task in my house would require a jackhammer.) There SHOULD be a P-trap below the drain. If you can get the tailpiece off of the bottom of the tub (unscrew the metal drain, then the tailpiece should pop out), you should be able to spear the plug.