Noisy belts showed the balancer is slightly out of true.
Doesn' t look too bad, but a replacement has been ordered.
A few days later:
Mounting the new Harmonic balancer made no difference! Still the belt made a very undignified noise. Spent another afternoon looking at all the pulleys and belt. Finally I disconnected all the mountings on the steering pump. Moving it sideways made the noise stop. So it seems the mounting bracket was bent a little. I could hardly see it, but mounting the pump with a few thin shims made a difference. Now I have a respectable no-squeak-baby-Wagoneer again! While I mounted the balancer I also mounted a new front seal, so it was a useful exercise after all.