Thanks guys. As I mentioned, I jacked up the rear wheels and put her in gear and I still heard the noise, so I think I can rule out the effect of the road on the tires and wheels.
I will troubleshoot for the U-joint and look for a gear tooth in the diff housing.
Is a u-joint replacement a very advanced procedure (do I need to worry about tolerances, shims, etc.)? I can't devote more than a few hours at a time to these efforts, so the total elapsed time between disassembly and reassembly can be many weeks, and I have a tendency to forget how things were put together.
