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Find a local shop that has the Hunter 9700 balancing equipment (many Discount Tire shops have it). If operated properly, it can balance wheels better than older equipment. And if it can't balance the wheels any better, there may be a problem with one of the wheels being out of round or otherwise damaged. Been there, done it.
Time to find a good tire shop....the Hunter balancer suggestion above is good,
I do agree with the dealer....I isn't the alignment. It is either a poor job of balancing, too much lateral run out on a wheel or a bad belt in one of the tires.
I'm assuming that the vibration is felt in the steering wheel and not a seat of the pants vibration, so one of the front wheels/tires is probably causing the problem. IMHO.