When your setting up a driveline you must take into account the angles in which your u-joints are operating. A “2-joint” shaft must have angles that are equal and opposite in order to harmonize the driveshaft. A CV driveshaft or “constant velocity" driveshaft harmonizes the driveshaft at the CV and the pinion angle at the rear end can be aligned with the shaft. Check out the attached video for a simple explination of driveshaft angles and u joint phasing.
Drive shaft velocity: https://youtu.be/gmV4qwLfOMY