Steering wheel shaking at high speeds?

Could be a minor out of balance condition with one of the tires, or a minor alignment issue that is unnoticeable at lower speeds. Also the tires may not be rated for the speeds you are attempting and are distorting.

Another possibility is a harmonic imbalance, the tread of the tires may create a noise that causes a wobble in the steering, the sound is always there but raises to a harmful pitch at a certain speed.