Thanks everyone.

The current problem is in fact with the parking brake gear. The right side just won't stay in place and the top half was clunking around in there. I disassembled, cleaned and reassembled. It's lessened a little but still very much there. It's all probably original and needs to be replaced. The shoes are pitted and worn. There is also a stuck piston on the same-side caliper. The differential is the source of the clucky sounds but appears to work fine with the brake disengaged. I'm old and cranky too so I can't criticize.

A few new questions:
I'm planning to get new p-brake shoes and hardware.
Any particular sources recommended for these? IPD seems reasonably priced.
How would one diagnose whether new cables are in order? It'd be better to save the extra $40-50 if its not necessary. But if it is, a functional parking brake is worth it, of course.
I rebuilt the front calipers a while back and it was a pain even with no stuck pistons. Who knows what's in there, if the stuck piston will come free, and if the piston and chamber will be salvageable. Any recommendations on a replacement caliper? Any reason not to grab a junkyard one if the pistons work? Also there are some A-1 cardones on rockauto that end up being about $25 after core refund and I don't have to yank off another car. Any good?

Oh, yeah, should L and R calipers be replaced together?