Need advice on tire balance issues

[motex]

Hey guys,
I just install today a new set of Style 214 replicas on brand new Toyo Proxes ST tires. (factory sizes 275/40/20 on front, and 315/35/20 on rear's)
The first time I drove the car on the highway I felt a strong vibration coming from the tires (not possible to know from which one), so I took the car back to the tire installer, he checked both front's, one gave us Zero reading and the second one gave us a reading that needed an extra 0.5oz of adhesive weights, so we did it.
Now that I drove back home, I still feel a vibration much softer but still vibrates, the wheels and tires are brand new.
QUESTION: do you think it will be better to check the balance of the rear tires too ? usually rear tires don't give any vibration to the car, or maybe the machine they are using is not well cailbrated ?
Any feedback will be appreciated !
Bernie