Well, I'm assuming that among 3 different cars and 5 different drive shafts, they can't all be bad.
I'm going to put my Dart on jack stands tonight and turn the wheels above 65mph to see if the vibration is there. I'll apply the brakes lightly to give it some resistance to simulate driving. Wish I had a dyno to do this, but have to make do with the means I have. If the vibration isn't there I'm going to remove the drive shaft and tow the car above 65mph. If it vibrates then, then I'll know it's in the front end and the bushings need replacing, which wouldn't surprise me at all as they are pretty old now.
