It's your endlinks. Tighten them well. You may need a zap of an impact wrench if you can't do it by hand.
As far as your shimmying goes, you need to get a good balance job done with a place that has their machines calibrated regularly. Not all balancing jobs are equal. You also need to remove the wheels and lightly sand all hub to wheel mounting surfaces. They need to be perfectly clean. You aren't using cheapo aftermarket wheels right? And tires, what condition are they in?
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