In reality the angle of any component (engine, axle, driveshaft..) in relation to horizontal is not at all important as far as the driveline geometry is concerned.
But let me explain,
Obviously you don't want the engine standing on it's tail or snout, it needs to be in the neighborhood of level so the oil system works, but there is plenty of wiggle room there, this usually is not a concern except in off road vehicles.
Now the biggest part of this driveline stuff you need to understand is the angles between the components are the important angles, it's all about the U-joints and the angles they work in. Yes many of the angles even in the PDF instructions are given from horizontal, but that's only because horizontal is a easy reference point, the actual angles relative to horizontal make no difference. Horizontal is only used to easily relate the angles to one another.
The same info applies to big trucks, small trucks, and even industrial (stationary) driveline systems using u-joints.
The one exception may be the torque tube systems like your 1/2 ton truck may have is probably not covered in the manuals. They only have one u-joint, and so long as it is within it's recommended operating angle (it was at factory ride height) there is nothing you can do with it.
As a final note, the driveline angles should be set, checked, and corrected when the truck, complete and ready to travel, is sitting on a flat surface (level is not necessary but can't hurt). If you'll usually be empty, then measure with an empty truck, if you'll always have 1,000 lbs of stuff in the truck, then measure with that stuff in the truck.
Grigg
