Redwood, and Hippie, you were correct.

It was a frozen rear u-joint, what a PITA this job was. I worked on this thing for three days.

Bentley makes it sound so easy, they say tap on the yoke with a soft faced mallet. No way, after beating and pounding, I was afraid I would damage something.

I did a search online and found a guy using a balljoint press, a giant
C-clamp with an assortment of cups and sleeves. This is the ONLY way to go.

I borrowed one from a co-worker, but this job still had pitfalls. You should be able to push the cups above the yoke, then remove them from the outside. That only worked on 2 out of 12, the other 10, I had to push them all the way to the inside. Then use a Dremel tool w\cutoff wheel to cut the caps, break them off with a hammer and screwdriver, and remove the needle bearings just to get enough clearance to remove the joint.

If you have pushed one of the cups all the way through to the inside don't be tempted to push it back up the other way. It will not stay straight, it will bind and damage the yoke. Also be very slow and cautious installing the new caps they had to be worked in using the balljoint press or a smaller 3" C-clamp, or they will also bind.

The other pitfall, don't buy CHEAP stamped steele snap-ring pliers. Under load the cheap pliers will flex and you cannot remove or install the snap-rings properly. The cheap pliers allowed one of my snap-rings to fling off into the weeds. So off to Sears for a set of good pliers, Craftsman with cast aluminum jaws, no flex, no problem.

Now for the added bonus, I think the U-joint had been bad for a long time, even before the vibration. This car always seemed harsh and stiff in the rear compared to my other wagon. I thought maybe the rear springs had been converted to overload springs. I think the bound joint was also binding the suspension, because now the car steps over bumps much easier. I took it over several speed bumps today and it rides like a new car.
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Bruce S. near D.C.