I've seen cases where it could even be tires (but don't ask me why we don't feel it when we're not on the brakes, i don't know that one). A Chevy work van is doing the same thing. Before changing tires, it would shake so bad when braking that it would knock stuff off the shelves, especially when braking down a mountain. With new meats, it has smoothed way out. The brakes were not serviced at this time.