The pinion angle should be 2 to 3 degrees lower (toward the ground) relative to the transmission output shaft angle. Of course it depends on what type of suspension you have...leaf spring, 4 link, etc. The "ground" has nothing to do with pinion angle, it's a relationship between the trans/engine and the pinion angle.:
When you accelerate the pinion yoke rises. When the drive line is under "load" (acceleration) the pinion rises. If the pinion is too high in the static position (no load) it will rise even higher under load causing a vibration. If the pinion angle is too low it will never rise high enough (under load) to properly align under acceleration. The trick is to get the drive line to properly align under acceleration.
