Just got back from shop - current diagnosis - after putting the car on the dynamo and crawling underneath to listen more closely - is that a bearing in the transmission is the culprit. Not replaced yet as insurance has to approve - thank god I notified them the day I got the car back that there was an additional problem.
My guess is that had this been part of the first repair estimate, the car would have been totaled.
I did present the idea that this could have been diagnosed by watching from the outside, and while this idea might have merit for other problems, the mechanic agreed that there is no way that this type of vibration could have been seen from the outside. Too small of an amplitude and brief duration.
Regards, S.