You'll want about 1/8" toe in. You also might recheck the steering box and tie rods for backlash as well, as both can cause the problem you're having. If you do want to add a bit of positive caster, the shims are available from Napa. They have a wedge shape, and you may need to trim them to size for width to match you're sping seats. Install them from the rear of the axle for positive caster.
