While what you did will find the center of the steering box, it will not correct steer ahead, that is a linkage adjustment.
Now, as far as things in alignments that can cause a pull, there's quite a few that can do that. Some you've already eliminated (tires, and some you really have little control over other that trying to compensate for them (road crown). We've already touched on Steer Ahead, and Jon mentioned Caster Spread. Both require aftermarket parts to correct on an AD truck. With Steer Ahead, and adjustable drag link, and Caster is the shims. You might be able to have a .5 degree shim put in the right side to correct for road crown. Camber can also be an issue, it'll pull to the high side, but unless the shop you're going to has the proper equipment to bend the axle, that's essentially non adjustable. Most shops that handle 1 ton and smaller trucks do not have that equipment, simply because it's been obsolete for 30+ years, most beam axle trucks still on the market use cam bushings for that adjustment. Steering Axis Inclination and Included Angle should also have been checked, while those generally will not cause a pull, it would indicate a bent spindle. The rear toe and camber should also have been checked. While not adjustable on these trucks, which is why many shops skip checking it, that can indicate if the axle is swung or possibly bent. Much of the vehicle tracking and stability is in the rear, not the front, a swung axle could cause a push, making it drive to one side or the other. The wheelbase check Jon mentions is a very good way to check for a swung rear axle, and, while it's using a different method, is how modern alignment equipment checks for that, as well as measuring the toe setting
