I have a 5spd TPI Formula and I get an annoying "buzz-like" vibration at over 75-80 mph. I changed to an aluminum drive shaft and still had the problem. I also bought new GTA wheels and tires, had them balanced, and it still wont go away. It does feel like its coming from the back of the car though. Also, it started right after I had my clutch done last year. Could rear wheel bearings be the cause? And how would I know?

 

Probably not axle bearings. They usually make a roaring kind of noise, at wheel speed.

Check your transmission's extension housing bushing, where the drive shaft goes into it.

It could also be pinion bearings, or simply, the pinion nut loose.