Check the engagment of the t-case, check the steering components, check that it shifts smoothly through the gears both in 2 and 4 wheel drive. If it has a body lift, check to ensure that it was done correctly, that it doesn't overheat due to the radiator not being relocated, and that any shifters have full throw.
Look at the ujoints, but realize they are cheap to fix, so not a big deal. If possible, inspect the ring/pinion setups for chips and whatnot, and to change the gear oil that is almost surely old and soupy.
Check to make sure it has a CAT, if it needs one, and demand that the owner smog it, unless it's off-road only stickered.
Minor things to watch for are tire wear (uneven tire wear could point to major alignment problems - once again, maybe not a big deal, but could mean busted wheel bearings or shot / modified torsion bars, etc.).
Most other things are routine maintenenace, and can be looked at after purchase, barring a blown engine or something of the sort.
If it was lifted with blocks, check the condition of the ubolts, and I wouldn't consider anything more than 3 inch blocks - they are just dangerous.
