After the new u joints and re=balancing, I have a hard time believing that the driveshaft is still responsible for the remaining minor resonant vibration at speeds above 65mph. With my 904 and 3.91 gears, there was no rear vibration before the driveshaft rebalancing. From there, I switched to a 3.21 gear with the same trans and no vibration. Next, to the current A-833 OD trans with 3.21 gears and no vibration, and lastly, to 3.91 gears with the current A-833 OD. With that, the vibration issue popped up, which improved drastically with the re-balance and new u joints. I suppose there could be some minor over-speed of the driveshaft, but it doesn’t seem like there should be.