I had a bad vibration that I knew was in the trans. It would happen at a specific rpm but that changed over time.
The mechanics who rebuilt the engine and trans gave me hell about it. FINALLY, I went to their shop, jacked the back of the truck up and removed the rear wheels. The mechanic got underneath while I revved the engine with it in gear. He immediately saw the problem. It was a bad output shaft bushing!!! I can't tell you how long it took me to convince this guy that there was a problem with the trans that THEY INSTALLED!!
Check that bushing....
