My 68 6 cylinder 3 speed manual drives as smooth as silk up to 60 mph. Beyond that speed, there is a general vibration throughout the car, some of it felt at the steering wheel but not excessively. In order to cure this, I was thinking, for starters, of having the four wheels rebalanced, then the driveshaft. If those steps don't cure the problem, how should I proceed? Opinions and recommendations would be appreciated. Thanks, Joe.