Driveline vibration help

Im looking for some help with a driveline vibration that's driving me crazy. I have a 68 Dart 270 4 door that I swapped the rear end in from a 7.25 to an 8.75. When it was done I had the correct shock pads for the 8.75 but was told they wouldn't fit so they used the pads from the 7.25. I have a vibration between 60 and 70 miles an hour. The car was put on 4 jack stands to try to locate the vibration, it looked to me that the slip yoke was bouncing up and down when it hits 60mph. The transmission was completely overhauled about 2k miles ago.The shop its in believes it was the bearing in the tail shaft that was causing the problem so they replaced it. Vibration is still there but not as bad but still irritating. Could it be the spring pads, would they change the pinion angles? Or do slip yokes wear out where they ride on the tail shaft. Im thinking about buying a new slip yoke and replacing the drive shaft with an aluminum one and hopefully cure the problem. Any suggestions would be very helpful and appreciated.