+1 on the tyres.

I had a vibration around 65mph, donuts were on the way out so I replaced them and the bushing on the ends of the tailshaft replaced and rebalanced the tailshaft.
Also had the wheels balanced, all were good.

Guess what? Vibration was still there.
Bought a new set of michelins and vibration gone.
I had michelins on the car but they were 10 years old and had started to go out of shape, thus causing the vibration.