My dealer tried to pull the old "it's your aftermarket wheels" thing on me, but it was the brakes. Do you have the brake issue as well? It seems that even the slightest warpage of the rotors will be felt if you've got larger wheels (20+).
I had the shimmy forever, but I went to discount tire after reading numerous posts from other people who had this problem. I can confirm that they were the only ones who stopped the shimmy for me, even though I had other places (and the dealer) do several road-force balances. Not sure if D.T. has the same machine, but the guy called it "ride matching" and it worked! No one else could fix it- I'd have the tires balanced, and within 200 miles it was back again. So far I've got 1000+ since going to Discount Tire and they're still fine. The only shaking I feel now is from the brakes (of course).