I was going to make that statement about rotors only causing shimmy when braking, but one should never say never and in thinking about it, the possibility exists for the rotor to be thicker or mounted on some debris in such a way as to put run-out into the wheel.
I have never seen it and I have seen scads of shimmies due to the soft lower control arms. But if he says it got rid of the vibration, I had to think of how it could do it. Thats how!
Bosch Master, ASE Master, L1
33 years MB technician