what causes steering shimmy
front wheel shimmy at 60mph on 2007 Chevrolet Aveo
Warped brake rotors or Bent wheel or tire extremely out of balance, Usually the first thing to do is have the tires balanced and rotated and they can check the brakes at the same time and by doing this it puts the front wheels and tires to the rear and helps narrow it down http://repairpal.com/directory?address=30068&car_brand_names=Chevrolet
Check the front tire balance too!
I've had 2 Chevy's ...your answer is simple, it is the tires. On my last Chevy, I was told to get a touring tire and I got those put on and the car stopped shimmying. I never believed it could be something that simple, but it was.