do you have owner's manual? suggested fluids are in there. i say that as they changed a few recommendations here and there and i don't know exactly about you 89.
some of the earlier t-cases had gl in them.. most latter had atf in them. etc.. if your truck is manual then i'd suggest just going to all the same GL4 <-- very important in the tranny, in all diffs, tranny and the TC. i went that way and like it as i only have to deal with one kind of oil.
as to exchanging these fluids: lay down under the rear pumpkin and have a look. there is bolt in the underside with a square hole.. a ratchet handle fits in there.. but first find one just like it that is higher on the diff. undo that one first.
same goes for the t-case and the front diff.. same deal there.. undo the upper bolts first..