[151fab]

If your tires are older than your front-end rebuild than they may have already been cupped. Once a tire is cupped it will continue to cup further, it will not "un-cup". If you have too much toe-in that could be causing the problems as well. You only want about 1/8".

Also possible, but less likely: your spring hanger and shackle bushings can be worn so badly that it would effect the steering play, a cracked or otherwise damaged leaf could also be a cause. Remember your pushing against the bushings/leafs when you steer because these are the only things locating the axle laterally.

You want to check all this first, before you start shimming in caster, because adding a butt-load of caster will hide all the other problems you might be having as well as wear everything out sooner due to excessive steering effort put on the system.