I'm just gonna say "it ain't the steering box." Tires following the road certainly is not the box
What have you done to troubleshoot the problem? My method (requires 2 people) is to have someone wiggle/ move the wheel some amount, which may have to change during the process, and watch the shafts/ linkages/ components from end-to-end looking for play. EVERYwhere. Play in the coupler, the splines, bearing radial as well as axial play, ball socket play in every rod end and the ball joints. Wheel bearings. And of course the tires themselves. And don't forget the steering box mounting bolts or for that matter any other frame/ steel/ structural member. Pay particular attention to the idler arm.
Have you read up/ done tests on "bump steer?" Without knowing the suspension height this might be a lot of the issue.
