Chased the same vibration for years 60-65, different diff, 3 tranny's, 2 used shafts out of other cars I owned that had no vibration, new DC drive shaft, eng mounts, trans mounts, belanced the eng, 2 flywheels, after 25 yrs I decided to live with it. finally a Schumachers poly trans mount cured it. I was getting conversion mounts for something else and thought what the heck, I'll give it a try. Smooth at any speed