Chuck,

MY vibration was at about 70 to 90 MPH. It was a droning or warbling, high frequency vibration. Tire vibration is MUCH more common and is at about a third the speed of driveshaft vibration.

If you're not sure if it is driveshaft or tire related, rotate tires and see if it feels different, if so it is tires, if not the next most likely possibility would be driveshaft.

That said, you need to think through a vibration before acting. If you kick it in neutral, does the vibration remain, if not it might be engine related. Does the vibration follow engine speed or vehicle speed?

Don't start doing things like I did without first determining that it is indeed driveshaft related.

Tires MUST be dynamically balanced on these cars. That means weights inboard and outboard on the wheels.

Good luck,