Well, we think we have figured out the source of the vibration in the minivan. We tested it again with different wheels/tires on it from another old van with no ride problems, and became convinced that it is the aftermarket aluminum wheels on the van that cause the vibration. We intend to replace the wheels eventually, but are not in any huge hurry, because driving the van around here locally does not cause a problem.

 

Our problem was finally solved when we put the aluminum wheels from another Town and Country on our van but had our tires mounted on them. We discovered that there was no longer any noticeable vibration. I still have the van and now enjoy occasional trips in it without incident.