I'd narrowed it down to the driver's side by sound, and so I started with the front wheel, on which (as you may recall) I'd recently replaced the brake pads. That one seemed to have some issues with the caliper sticking (it's the other one from the one that had serious problems, though), but it seemed to not really be making that much noise. Mental checklist adds a note to do something about the front calipers sooner rather than later, and I put things back together to check out the back wheel -- which, I note when I get it jacked up, does seem to be a bit hard to rotate, and a bit noisy.
To make a short story shorter, the lug nuts on that wheel seem to be deeply stuck. Remarkably deeply stuck. Specifically, this stuck:
(And, no, I wasn't using a length of pipe or anything on it to loosen it, either! If you can't make out what that is in the picture, it's the end of my lug wrench, twisted off and very firmly stuck on the wheel nut.)
And, to make matters worse, that end of the lug wrench is stuck on there pretty good -- even beating on it with a hammer didn't budge it a bit.
I'm not really sure what to do about this, at this point. I guess find a brake and tire shop to take the wheel off and figure out what's wrong, but ... the only brake and tire shop I know and trust is 2500 miles away. Beyond which, I'm a bit concerned about driving this 2500 feet with the lug wrench end on there waiting to fly off if it does work loose.
The above was written last night, actually; I just hadn't gotten around to posting it. Today, I carefully drove the Bronco up to the brake and tuneup shop a couple of blocks away, and they fixed things -- which amounted, as it turns out, to replacing pretty much the entire rear brake system -- pistons, drums, and pads. It's much better now, and I'm muchly relieved not to have to worry about it.