2002 Sierra

My experience is that shaking and viberations are often related to the tires and wheels - but not always.

However, since yours becomes worse with higher speeds, it would be good to start with the most common cause.

It's cheap and quick to go to your tire shop, have them rotate and balance the tires.

If it still happens after that, you might check the steering linkage and suspension. See if there's any freeplay or loose ball joints.

LEBoyd