Rear end not centered

Welcome to one of the issues with leaf spring rear ends. Any distortion or damage to the spring mounting along with normal factory tolerances (or lack thereof) can create this issue.

The only way to prevent it or keep everything centered is with a panhard bar or watts link. But before slapping one of these on there, you might want to check to make sure everything is square (cross measure from common points with the front suspension to keep from "crab walking") and that by "influencing" the rear back to where is should be you are not putting anything in a bind.