There were shims on both sides. A thick, solid one on the driver's side and three thin ones on the passenger side. I put them back as I found them. Which says very little, but may be the key problem. I vague recall some old posts where people asked which sides the shims should be on. I seem to think both sides was bad. I've got spares in my stash if I need more. Perhaps I should get the shims off the other side and put them on a single wheel...?

This is one of the short-list of amazon components I'm still a complete novice. I've generally avoided the issue altogether and I've not investigated this as a cause of the problem at all.

Thanks for the info, I'll see if I can puzzle it out.