Driveline vibration!!! I need help!

I think we need more info in terms of the vibration... If the car is in nuetral, and you rev it slowly up to 4K rpm, then back to idle... Do you feel a vibration? If yes, it's in the engine/torque convertor. If no, continue:
Drive the car. What speed does the vibration first become evident? Does it start up, get worse, then go away as you slowly speed up? Is it in your seat, or in your steering wheel? Is it constant or does it sort of resonate in your ears (think resonate like the rumble you hear when you drive with the windows open on the highway...)? Last, get the car to the speed of the vibration, then go a little faster, and carefully slip it into nuetral. Does the vibration go away as the engine comes to idle, or does it go away slowly as the car glides and loses speed?