How hard is it to change the rear transfer case output seal? Mine is seeping a little bit. I'm changing the u-joints today so I'll already be in there.

VEry easy, see the following page for pinion seal replacement, essentially the same thing, just no worries about pinion bearing preload, etc.:

http://www.4crawler.com/4x4/CheapTricks/PinionSeal/index.html

Only other tip is to apply a bit of silicone to the splines before reinstalling the flange to seal up that area.

30 mm impact socket and a 1/2 cordless impact makes short work of it. A seal puller tends to help more than the screw driver method to remove it.
Definately recommend getting one.

Sounds easy enough. Is it also 90 ft lbs torque?

I think the FSM calls for 90 ft.lbs. That is usually what I use. Short of stripping the threads on the output shaft, you probably can't have it too tight, within reason. I know when I first removed mine (to redrill the flange) after having it built by Marlin, I needed a 4' pipe over a breaker bar on the nut and a similar bar bolted to the flange to keep it from turning to just barely get it loose. This was after two different 1/2" impacts failed to get it loose. I say I just barely got it off because the pipe on the breaker bar was bending under the torque to the point it was bent from the frame to the ground before the nut finally gave way. I'm thinking Marlin may have used his special impact gun when he put that t-case together <img src="/forums/images/graemlins/smile.gif" alt="" />