What your describing isn't really bump steer which is the anomoly caused on "side steer" set ups when the drag link and radius rods move in different arcs during axle travel. Cross steering is, however, susceptible to the axle shifting on the shackles in corners, and that movement is fed into the drag link causing the steering to over react to input. If you're just trying to get rid of the steering problem, why not just install a Panhard rod or a "dead spring perch" on the front axle? That'll stop the lateral movement and eliminate the problem.

Bob