I'm going to make my follow the car recommendation again. You can do this yourself or with the help of your mechanic, but you will need another vehicle and driver.
Find a road with very little traffic. Either have someone drive your car while you follow in another and at various speeds, say 25, 45 and 55 mph, get a look at the passenger rear tire to see if it is bouncing or wobbling. Also check to see if the car is "doglegging" or "crabbing", that is where the car is not quite pointed in the direction it is going.
You could drive your car and your mechanic could follow in his. The mechanic might be able to spot something that you might miss.