Excellent. It's good to see a conclusive diagnosis to a problem like this, rather than the approach that seems to go the route of do more alignments, rotate the tires, do more alignments, do four wheel alignments, do more, rotate thetires again... ad infitum. So much effort is wasted in these types of efforts. Four wheel alignments being one of the more abused things.
Most cars that can benefit at all from a rear wheel alignment can only allow for a camber adjustment in the rear. Camber is not the culprit in most vibration related problems, yet too many owners suffer through, or worse... pay for multiple four wheel alignments in pursuit of problems like this.
Hats off to your mechanic. Go buy that guy a beer.
Isn't it a shame how many tires are so much better than Michelins, at far lower price points?