Sad but true. The last four years have not been fun for the trailer industry and many companies are 'shaving the edge' of things too much. With 3,500lb axles, i would say that it is very likely your right rear spindle has gotten 'tweaked'.
It happens a lot more often on the right side than the left, because people have a tendency to hop over curbs with trailers while turning right. Go find something extremely straight and lay it front to back across the faces of both wheels on that side. If the spindle has gotten a little bent, then the tire will not be straight with the front one.
The funny thing is that i usually have people bend the FRONT right side spindle. Doing on the rear is a little unusual, but well within the realm of possibility.
Hope this helps.... now, go spring for a new set of steel belted st205/75R-15 C rated trailer tires and you will be good to go.
Craig Hull
AKA the trailer guy
Stateline Trailers
Thanks Craig. I'll check the wheels for being parallel this weekend along with Chris's suggestions.
I replaced the rears with Radials and ordered two more for next week.
Bill