what pinion angle?

Supershafts said
Put the car on stands or on a drive on lift....

1. measure the angle of the driveshaft installed in the car...
2. measure the angle of the pinion or across end yoke

3. set the diff 4.5 degrees opposite the driveshafts angle.

Now when you are under hard load, the rear angle should be 0 or .5 degrees and you won't have additional loss thru angle.

What you neglected, here, is the TRANSMISSION SHAFT angle.

What you ideally want is when the car is under hard accelleration, you want the PINION shaft and the TRANSMISSION shaft PARALLEL

Just like someone else posted:

http://www.moparts.org/Tech/Archive/axle/8.html