A friend has a mud buggy with r&p steering. The rack and steering box were mounted to the axle, with a pair of proper velocity sensing hydraulic steering dampers on the tie rod ends. It almost eliminated bump steer, but he had to have a special splined telescoping steering column fabbed. The part of the bump steer you cant get rid of is the part when the axle twists over extreme single wheel articulation. That part is there whether the box is on the frame or the axle, but the steering gear ratio multiplies the effect when axle mounted. It does get rid of the bump steer caused by simple vertical movement of the axle which usually puts toe out on the wheels as you jounce down, and toe in as you rebound.