130,000 miles,well maintained......

Wacky and disturbing side to side shimmy or shake most noticeable when starting from rest,disappears around 20-25mph somewhat.

More noticeable when riding in the backseat.

Floorjacked it for the guy,rear wheels are straight and not out of true.

Could it be driveshaft related?

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Nothing in the driveshaft or drive axles can create enough force at those speeds to be the culprit. If they did, they would shake your teeth out at higher speeds.

My first thought is to carefully check the rear tires for an out of round condition. You will feel this condition dramatically in a M/B, generally at about 10 mph or so.

BTW, you cannot normally see the out of round condition with the naked eye unless it is very bad. Use a piece of chalk and see how much it contacts a moving tire. You need a fixed frame of reference for a reliable judgment.

Yeah, I know what you mean. I'm more accustomed to 28 to 34 inch tires on the antiques I mess with where out of plumb is really noticeable to the naked eye.

I thought about rigging up some sort of fixed indicator.

I'll tell him to have the wheels checked and balanced,then I'll take it from there.

Just thought I might luck out if this was something particular to the above model,Thanks.

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I would do a front to rear tire rotation. Could eliminate or condem a tire quickly. Do one side and drive car. Then do the other side rotation and drive if first side rotation did nor identify the tire.The defect if tire rotated should be more pronounced when put on front. Thinking broken belt causing effect perhaps. The balance job may not detect or change the problem. If nothing else is going to condem or clear the tires. Also could consider using the spare on one wheel and moving it eliminating one complete wheel at a time. That way the problem if present in one tire is going to be found with absolute certainty. Also eliminates the possibility of a bent wheel. The major problem with the problem being detected on a wheel balancer is they lower a shield prior to spinning up the wheel for safety purposes. But if informed before the balance may check out each unit a little more carefully. Think brian is right about it being tire related.