So our 68 300 has a high-frequency vibration that comes and goes when driving down the road. I just recently had new springs installed and KYB shocks at all 4 corners, so it's sitting at stock height now but that didn't do anything to make the vibration better or worse. The garage we took it to said they saw some dents in the driveshaft and the exit shaft at the transmission had some play in it, but they said the U joints seemed good. The transmission itself seems to perform great with crisp shifts and the speedometer is way off (reads 80mph when you're going 60) which makes me wonder if it was swapped in from a different car with a different rear end ratio. I notice that sometimes it seems to be dependent on driveline load (i.e. accelerating or decelerating) so if you modulate the throttle to remove the load, or drop it into neutral, the vibration will go away almost completely -- though not always. Sometimes it does not go away, which confounds me that's it's inconsistent like that. It is not dependent on engine RPM and it does not vibrate when sitting still in neutral and revving the engine. The frequency of the vibration feels too quick to be a tire. Anyone care to take a stab at guessing where the vibration could be coming from? There's a shop in town that specializes in transmission and driveline work, but I'd like to have at least some ideas of what it could be before I take it there. Could it be from the play in the exit shaft? U joints? Rear end? Driveshaft?