I have a 1988 IROC modified throughout rebuilt 700r4 . When I'm cruising in Overdrive at about 50mph to 65mph I feel and hear vibration. It doesn't seem to get worse the faster I go, and it also seems to stop when I'm over 65mph. I had considered it to be unbalanced rims or drive-shaft.
Was wondering what you guys think?

 

Don't forget to check your tires also. An out a round tire, lost wheel weight, broken belt in the tire. Vibrations are a bear to find. Bent axle hub, u joints, bad shock etc. For what it's worth, I changed out my locking lug nut on all four wheels and my vibration is not as bad at 70 anymore. I don't know if the little extra weight of each nut made a difference or not, but the vibration i had is not noticeable now.