Originally Posted by guyhatesmycar

I'm thinking it's a rear axle issue, as the front end alignment would have discovered any issues, and the rear isn't really looked at on a solid axle, leaf sprung system.

Do you have any broken springs? My 83 Toy shortbox usually breaks one every year or so, and that can wildly affect handling. Also, you should have a friend drive your truck, and follow behind it, watching for dog-tracking, where the front and rear wheels aren't following the same path. If it is dog-tracking, the ass end of the truck will appear skewed to one side. This is a common problem after frame or suspension damage.

Good luck.