[Laird Scooby]

What rear suspension do you have on yours, is it Nivos or normal?

Either way, weak rear suspension can allow the back end to sit lower causing vibration not only in the prop centre bearing as Phil mentions but also the diff nose bearing - it's all to do with things not being lined up as they were designed to be.

Several people who have lowered their cars get this problem too.

If you have Nivos, i'd suggest new rear springs and shock absorbers/dampers would also help, if you have the standard set up, look to see if the back of the car is lower than the front when unloaded and on level ground. If the back is lower, renew the springs and shocks.