Put my vote down for balancing. I have had multiple times where I just picked up the car from the shop (who used a Hunter machine) and there was still a shimmy at 70mph. I think if you sneeze on the wheels it throws them out of balance so I suggest taking the car back and getting them balanced until they get it right.
BTW-Both times it was on 18" wheels both OEM and aftermarket on my 2002 S.
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