based on your description I would guess a problem with the output shaft support or govenor support.
I have seen vehicles with your symptoms; upon disassembly of troubled units, have found very deep grooves worn into these housings where the sealing rings ride. Might be okay for first couple of shifts when cold but once warmed up, would do what you describe.
You can leave the trans in the vehicle for this, but have to remove transfer case and adapter for govenor and/or housing inspection
e-guess for trans e-repair