Klaus: I appreciate your suggestions and followed up on the "road force" balancing. Turned out to be a long-drawn out adventure. I finally had my old Volvo tech go for a drive with me and he agreed there was a problem. A hoist and a bit of old-fashioned wisdom quickly identified the problem as a dent in the drive shaft. If I knew how to do it I would attach a picture.
Howcome it took three sets of tires, and about five techs looking at it to finally figure out the problem? Too much reliance on computers and not enough on common sense.

Anyway, the problem is fixed and the car runs nice and smooth now.