I don't believe we have ever discussed the reason for maintaining a parallel line through the drive train here before?

As universal joints turn, they speed up and slow down. By maintaining a parallel line with the trans output shaft and the rear pinion, the universal joint at the trans output and the pinion cancel each other out.

Say the trans output shaft is at a 3 degree angle down, and the pinion is at a 3 degree angle up. When the output tranny shaft joint speeds up, the pinion joint slows down and vise versa, cancelling each other out. The same applies if the trans output shaft is off center of the pinion.

Dean