Mike,
No doubt that is what you want in a race / drag car.
Most highway vehicles will spend almost zero time twisting the diff significantly, particularly with the trailing arm suspension on these trucks.
The point is well taken though. It doesn't matter whether the diff is nose up or nose down as long as the working angles of the universals is correct (front and rear close to equal, at least 1 degree and less than 3 degrees)
Stock set-ups are typically engine tail down, diff nose up to get the universal working angles correct. I think the trucks use a higher nose up angle to allow for loads in the bed which will shift the working angle towards a smaller number.
Here is a neat tool for measuring pinion and drive line angles. I calibrate mine using a good level.
http://www.harborfreight.com/cpi/cta...emnumber=95998
TR