I'd suggest a toe-in adjustment of 3/16" or so, measured after the truck has been rolled forward a few feet on a flat, hard surface. Do you have any way to check caster and camber? Unless there is significant wear to king pins and/or bushings, the camber should be close to correct if the axle isn't bent. Camber problems usually create a pull to one side or another, not a problem with wandering back and forth.
You're most likely encountering a situation where there's too little caster, the angle the kingpin makes with vertical looking at it from the side. An example of caster is the front fork angle of a bicycle- - - -tilted slightly forward at the bottom. This makes the steering try to return to center after making a turn. If the kingpin is too close to vertical, the wheels will wander around aimlessly, rather than trying to return to a straight-ahead condition after a turn or a minor steering correction. A tapered shim can be inserted between the axle and the spring on both sides to increase the tilt of the kingpin and assist the return to center after a turn. Too much caster makes for hard steering, so it's definitely possible to have too much of a good thing!
Jerry
