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If both edges are worn and the center is good, it's just low air pressure in the tires. That still won't give you the shakes.
Make sure you jack it up at the frame and not the axles. Some of the shake may be caused from a bump or pot hole unloading the suspension and then the weak link, wherever it is, is causing the shake. Get all the weight off and whatever component is causing the problem will be easier to find.