The guys have just about nailed it with their suggestions but I'd like to add a couple more. My 54 straight axle, radial tires at 32 psi, with slightly loose king pins, tracks straight as an arrow and is a pleasure to drive at 65-70 mph. The spring bushing fit is critical and if everything else is tight I'd consider a check of the toe-in. I added a tad extra toe and a steering stabilizer (shimmey dampener). I run no caster shim. Just want to clarify that slightly worn king pins are not necessarily going to cause your truck to bump steer and wander. Replacing the king pins isn't necessarily going to cure the problem! It doesn't cost anything to add a little toe and if your spring bushings are in good shape you might just eliminate the issue.
Dave
