Two possible causes: 1. Faulty tire 2. Rear wheel "cocked" in frame. Park it on the center stand and sight up the rear wheel to the front wheel to see if either one are off vertical. I do not know if your rear wheel is one shock/spring or two. One weak spring or shock in a two shock/spring system could shift ALL the static and dynamic loads to one side. Test for this by removing both shock/spring units and compare how they feel as you carefully "plunge" them up and down on a block of wood, rubber pad or something that would keep them from slipping on the floor. The single arm, single shock set-ups would have some structural or mechanical problem that would cock the wheel. A bad tire may be inspected after removal. Let us know what you find and maybe we can come up with some other "bright idea" on what could be wrong.
Karl