Bump steer can be a problem on solid axle vehicles. The wheels don't articulate inividually as they do on an ifs, so the forces often are tramsferred to the opposite wheel. Power steering does help to aleiviate/mask the problem, especially the slave cylinder type that has a ram directly on the tie rod. Another fix is to add a steering stabilizer. I opted to do that on mine. Moog makes a good one that will fit pre-'59 solid axle trucks. It's a Trail Boss damper, part # SSD12. You might have to do a little fabrication to make it fit perfectly, the mounting hardware is intended for a 4x4 axle. The mounting is similar to the slave cylinder for the factory type power steering.
