1994 Safari Tranny Problems

I have a 94 Safari that is having some shifting problems. Im curious if anyone else has had this problem.
Problem is once I hit 50-60 mph when it should shift into O/D the tranny starts to "hunt" and it cannot decide to stay in O/D or shift down. Also the speedometer will "drop" its registered speed until I release the gas pedal and it will resume "proper" reading. Doesnt seem to have any problems in city driving that I have noticed. I had brought it into a shop, and the output sensor from what I was told was bad, and was replaced and that was what was causing the problem, but tonite after picking it up from the shop, its doing the exact same thing all over again. :confused: Any comments or insight on this problem would be greatly appreciated.