vibration problem

There's a bushing in the tailshaft of the TF. It's probably shot and causing your problem.

I had a bad vibration in my Dart. Tranny needed building so I pulled it and sent it to the trans guy. Got it back and was going to test drive it and check pinion and other things to get rid of vibration but when I drove it all the vibration was gone. It was what 75sport said. Bushings were shot in trans.