Vibration

["Mike Ashby . Mathematics" <ashby@smhs.medford.or.us>]

Greetings,
 I have a vibration problem in my '80 B.It occurs mostly when shifting
or at higher speeds when I let off of the gas at higher speeds.(55-65mph)
I've had the car on a lift and the exhaust system is solid.The clutch was 
replaced about a year ago. I checked the motor mounts and the left front
has a crack ,but I cannot see any movement there when I rev the engine.
Your diagnosis of probable cause would be greatly appreciated.

Godspeed,
Mike