what motor do you have in it. If it is the 4.3L then you should have no problem bolting up any 4l60e trans (or 700r4 if you don't want electronics). On the other hand, if you have the 2.8L i think the bell housing bolts are different from the standard bolt pattern so you'll have to find a trans off of a 2.8L.

As far as going from 4wd to 2wd, my buddy did the opposite swap and it worked fine for him. He has a 2wd s10 and found a 4l60e with a t case and bolted it up and so far he has had no problems. So i don't see why going from 4wd to 2wd wouldn't work out.

Best of Luck!
-Tyler