[rustbucket66]

It sounds like you have to change the pinion angle a bit. If it is smooth when you have it at a higher ride height, that eliminates any problems that may be in the driveshaft. When you drop it down, the angles change and you get the vibration. You can buy shims from the driveshaft place you were at, or try to make some. I think the pinion angle needs to be angled up one or two degrees. The shims go between the trailing arm and the axle mounting block. From what you describe, I think that will correct it.