Driveline vibration on 64 cuda

If the driveline is factory original, then a '64 would have a ball and trunnion transmission and driveshaft. After 60 years, things are getting pretty worn (I can pretty much guarantee that even if the rear U-joint has been replaced, nobody has even touched the ball and trunnion front joint). It will take some hunting, but rebuild parts are out there. As a temporary solution, try putting some 1/4" spacers between the transmission flange and the trunnion housing- this effectively moves the balls to an unworn (or at least less worn) section of the housing, and may eliminate your vibration. Oh, clean and repack it with good grease while you've got it apart.