Unfortunetly, tryin to diagnosis vibration issues is sometimes like catching a piece of straw in a hurricane. Tires and wheels would be my first guess. If you feel confident that your wheel and alignment shop have done their job properly, then I would probably move on to the drive train. Wheel bearings, differential, drive shaft etc. It maybe helpful to put the rear of the car up on a set of jack stands, putting the car in gear and running up the RPM's a bit.
Good luck in your quest !

Just my Dos Centavos !