Before I did anything I'd move the rear tire to a front axle....set up a block of wood, spin and determine that the tire is not out-of-round or that the rim is bent. Also can check rim runout with just some simple blocks and a pointer. Once you've determined that the rim is not bent and that the tire is round, you can move on to the more complicated stuff. Place the "confirmed" rim/tire back on the rear axle in question.....if it's bent you'll be able to see it....you know the rim/tire is true. I've seen rust/road gunk on the wheel mounting surface throw off a rim. It takes a pretty good whack to bend an axle.....rims and tires are much more often the ones getting the beating.
I always try to do the least complicated troubleshooting first.....getting a firm baseline more often than not...leads to a simple fix. Hope this helps.
Dave
