That reminds me of a car vibration problem I had at speeds above 65mph on another car this year. I was inventing in my mind all kinds of problems it could be. I talked to a mechanic who said most of the problems of vibration at speed are tire related - balancing or other problems. I took the car into my tire dealer to balance and they showed me that one of the tires was worn down to the steel belt. Put new tires on and voila - no vibration.