I have a 1992 XS V6 Auto. Trooper. It has suffered from that unexplained "graunch, transmission slip", from a dead start since I bought it (used).
After reading a few posts here about the possible cause being degraded rear differential fluid, I asked my mechanic last week to replace the diff oil with the best available. He did, and the problem has disappeared!
He used an American oil , so it wasn't familiar( I'm DownUnder). I think it was MUTROL, or MUROL, something similiar. I'm sure you guys stateswide will recognize it.
If you have this problem, can't hurt to give it a try.
Johnno