I topped up the rear diff fluid (about 1/2 quart), and of course it made no difference. I stopped at an empty school parking lot to see what I could see, braking in forward and reverse, and then I noticed a new, louder noise that was definitely wheel speed related, and on the right side. So I checked the front wheel bearing and it was fine, and then I jacked up the rear axle. It sounded terrible, with something nasty and crunchy going on in the right rear drum.
It turns out the adjuster cable had broken and the end was busy grinding itself away on the drum:
I picked up a hardware kit at AZ and fixed that, and now all I have is the original higher speed noise, although it is fainter. There is some notchiness, but it is hard to tell if that is the diff or the pinion. Perhaps it's a sign of issues with the pinion shaft bearing, but it's not bad yet and I'm going to let it go for now and keep and eye/ear on it. Overall the rear is quiet while running and the noise I hear is very faint, only audible sometimes with the rear window open when almost at a stop.