It's my pleasure to help, and I always learn something because of all the questions I have to ask in these threads. So here's some more... It's possible this car had a bad front u-joint, which trashed the tail shaft bearings in the trans. The joint got fixed but the bearing got ignored. But if that were the case it'd be leaking a lot of oil. The trans and drive shaft would be all oily and so would your driveway. You did check the trans oil level right? It does this regardless of what gear you're in? Are the rear motor mounts both intact and solid? Does this vibration go away over 45 mph? How fast have you driven the car above that? I'm thinking you might have a harmonic balancer that's "slipped", or maybe the wrong flywheel for your crank. Some of these questions might sound stupid, but I once asked a guy whose car was overheating all the time if he noticed the water level dropping in the radiator. He said he didn't know because the cap was too rusty to remove. D'OH!