I have a 1996 K1500 silverado Z71 off road with a 5.7 and 4L60 in it. I had the transmission rebuilt and it works fine. But I guess it wasn't my problem to start with. The truck driver fine at 40mph and below. Or in other words, in 1st thru 3rd. But once it shifts into overdrive, and you unload the transmission ( ie: let off the gas or top a hill ) the transmission bucks hard. Only thing I can compare it too is the transmission shifts into reverse for a split second. It is hard enough at highway speed to throw stuff off the dash board.
All help is grateful....

 

Bought a reprogramed PCM hope this solves the problem.