I played around with it a little more and what I found out was that if the seat belt is pulled ALL the out to the end, the mechanism makes a slight ratcheting sound when it retracts, and locks up every time, no matter where it is. The solution seems to be to remove the belt from the guide on the seat and let it retract all the way, then it operates smoothly and quietly again.
My guess is that’s exactly how it’s supposed to work, and I just never noticed that.
Or maybe the mechanism is on its way out. It is 23 years old, though it only has 73,000 miles, and the previous owner’s usual passenger seems to have been a large long haired white dog...
Anyway, it’s working again more or less, though that may change