Swap tires left to right and see if the pull follows the tire.
I found my axle had some wiggle room in the locating holes for the spring pins, and shifting the axle would change the steering wheel location. Yours could be the same, one side could be forward and the other rearward causing the truck to want to go around a corner. Loosen the u-bolts and pull the axle forwards on the side the truck wants to turn to, or pull the opposite back. Rear axle could be the same way, slightly crooked.
Drop a plumb bob from each corner of each axle from the same location and mark the spot on some tape on the floor, measure criss-cross to see if its square, then front to back to see if its even side to side. Be accurate with the plumb bob or you will be chasing your tail for no reason, I've been there before!
