Pinion angle

That video is excellent. The "simple story" is that with a common 1 piece shaft like A bodies use, the two end shafts (trans and pinion) MUST BE PARALLEL under load. You will find a 4x4 does the same thing. The shaft speeds up and slows down every 1/4 turn, and that is cancelled at the "other end" IF THE angle is the same at both ends.

You can "feel this" for yourself with a socket drive U joint.