Interesting comment about the right rear. The shop is well regarded, at least within MBCA. I believe they checked all bushings and links (all of which had been replaced, anyway), they checked to see if steering was too tight, tire pressure, and noticed a small amount of feathering particularly on my front left tire.
My front left tire typically shows a bit more feathering than any other tire, and the rears look like they're wearing perfect.
It may just be mind over matter, but it seemed that on my trip to Montana and back (6300 miles) that the front left OR right tire would show a bit more wear from time to time, possibly because of the very strong winds and I was really holding the steering wheel to keep the car from blowing into other lanes.
Either way, I'm going to replace the rears (because one is leaking) and probably the fronts as well. In part because I'm just tired of them and possbily because they'll solve the problem. I expect to do the work in October.
Thanks for the suggestions,