I had a vibration also that was there in nuetral and driving. And after going thru all possibilities, balancing shaft changing ujoints, and going thru convertor swaps and trying the different convertor and plate combinations as we thought it was the convertor, it ended up being the motor mounts. Changed the motor mounts and vibration was gone. And after I changed them, we had a hard time finding one side, I found a few pairs of NOS pieces.
Anyways, check your motor mounts, even if you changed yours recently as the aftermarket jobber store ones tend not to last (if you can find them at all now at the jobber stores).