Back ground: Purchased a 75 Bronco that had rotten tires and the 302 and C-4 was worn out. Never drove the truck over 35 mph when I decided to take it down for restoration. I have a similar issue, but mine does not start until 51-52 mph. Up to that point it is smooth as glass. It shakes so violently that you have to slow down. New drive shaft, engine, harmonic balancer, flex plate, torque converter, motor mounts, pulleys, C , radius arm bushings, and trans/t-case mounts. The ball joints are good. Replaced a rear axle that was wobbling thinking that would be the fix and it did not. Nope! C-4 rebuilt that did not fix the problem. Checked the drive shaft in another bronco and it was smooth. Pulled the drive shaft ran the engine up to 75 mph and no vibration. So under a load from the rear end it has a vibration. The t-case works fine and the rear end has the typical wear. Another bronco friend who has owned 3 over the past 30 years is at a loss as I am in determining the root of the problem. It is getting frustrating. So do not feel alone m_m70.