If your Jeep has a standard driveshaft (one U-joint at the front, one at the back), then you need to set the pinion angle so the U-joints are in phase. What the heck does that mean? The axis of pinion rotation and the axis of output shaft rotation must be parallel. What the heck does that mean? Well the U-joint caps travel in elliptical paths as a driveshaft turns at an angle, so the caps change speed constantly and vibrate. If the U-joints are in phase, or at the same angle, the vibrations cancel each other out. If one is out of phase, both will vibrate. Again for a rear axle with leaf springs, you want the pinion to be down 1 to 2 degrees to compensate for pinion angle change when under load.