Reply to "Driveline vibration after lowering car?"

U-joint vibration differs from the normal vibrations like an out of round or unbalanced tire. The vibration is torsional, that is;

While one flange is spinning at a smooth speed, the other flange turns faster for a ¼ turn, then slower for the next ¼.

During acceleration, the rear end is turning “smooth”, though increasing speed, where the tire is spinning with the jerking rotation.

How much of that jerk you feel in your seat depends a lot on tire stiffness.

During coasting, while your seat my not feel anything, the smooth spinning tire is now banging the gears in the rear end