1997 Suburban transmission

Ok I am working on a 1997 suburban 1500 350 engine in it. The truck runs great but after it heats up the trans doesn't like reverse. I think the fibers inside are gone and the owner decided to replace the trans. When I called around I was told that 1997 transmissions are the only one that will fit supposedly G.M. used it one year which sounds ignorant to me. Whats different on this transmission? or does anyone have any clue what else to check before replacing? thanks

You should be able to use a 96 at least up to the new body style, 95 was supposed to be the only 1 year tranny