Do you have a picture of the bottom of the front end showing the mounting points of the two lower a-arms?
On the rear axle, you need to determine if the tire / wheel is the problem, or the axle itself is not centered. Swap the tires from side to side, this would be a good time to measure the axle location when the tires are off. Measure from brake drum to leaf spring and/or leaf spring mount on the axle.
The only way to tell it's centered is to start measuring. I recently centered up a Trans Am front subframe and rear axle assembly. I started with both ends of the car setting level on jack stands so I could get under it. I used string and a plumb bob and dropped points down on to some masking tape on the floor. I then meaured from the points on the floor which is much easier then trying to measure the actual car. The more points you drop, the more accurate you can be, check the frame also to be sure its still square.
Go to the GM Heratage Center and get a copy of the information packet of your truck, it will have frame measurements you can use to help determine whats what.
Joe
