Pinion angle is a little more complicated to check than most things. You might want to have a shop do it.
Basically, the rear end of the transmission tail shaft is probably not going to be at the same height as the input to the rear end, so the drive shaft has to run down at an angle from transmission to rear axle. Oddly enough the angle of the rear end input shaft is usually not horizontal. It will usually be angled upward 4 or more degrees toward the front of the truck. I've always assumed that this is to reduce torque effects, but maybe someone can give us the real reason.
A big related problem is driveshaft U-joint phasing. For an explanation a lot better than I can give, check
http://en.wikipedia.org/wiki/Universal_joint in the Angular Velocity section.
If someone has buggered up your drive shaft, such as replacing yokes, this might be a problem.
Bent wheels have been a problem for me. My 1973 Chevy C-30 had some unusual wear on the tires when I bought it, so I went to the local truck tire place to get some new ones. They noticed that both of my front wheels were bent, quite noticeably. The symptom was a shake in the front end between 47 and 55 mph. It was fine at all other speeds.
