I had an early Mustang that had a balance problem. I spent a lot of time and money trying to locate why. Tires and wheels were checked for true and balance. The driveline was rebuilt, and balanced twice. We even replaced the flex plate in the transmission and still the problem persisted. I finally found a mechanic that resorted to using a hose clamp and a bolt on the driveshaft, moving it a small amount around between test drives. He got rid of all the vibration. He said it was coming from somewhere besides the driveshaft. His fix was there when I finally sold the car. I did tell the new owner that although it looked funky, not to mess with what looked like a farmers fix! True story and I still can't figure where the vibration originated from.