The driveshaft on the 90-93 Q45 had a thinner rubber bearing than the ones found on the later model years. According to my Q45 specialist, it was a common issue and was usually replaced under warranty. If your Q45 has 100k+ miles on it, I would suspect the driveshaft bearing. Purchasing a factory driveshaft now would most likely get you the later model shaft with the thicker bearing. Another issue could be your transmission mounts and exhaust hangers. The rubber may have deteriorated and severly hampered it's ability to dampen the vibrations.