Oh heck, I figured it out! The hub shaft didn't move, it was the collar just behind the wheel that had come loose, with one bolt missing. The wheel was moving inward. I jacked it up, pulled the rear wheel off and then found two of the three remaining bolts on the collar were losse. Tightened it all up where it should be, and put wheel back on. Looks good now, although I'll have to buy that fourth bolt.
And it's 100 F out here today!!!