[ss3011]

The Engine/Transmission shafting needs to be parallel to the rearend pinion shaft. This will allow each of the u-joints to run at the same angle which cancels the acceleration and deceleration the driveshaft sees each revolution. That is why the u-joints are phased to each other. If you crank too much pinion angle into the rearend, then the u-joints will get out of phase, and you will have a vibration. The pinion angle will change as the rear suspension moves because on a 4 link the upper and lower bars are typically not run parallel. Might be worse in this case when slowing down.