have someone sit in the truck and rock the steering back and forth with the truck sitting on the ground. Meanwhile, look under the truck at ALL of the steering/linkage. Start at the ends and work your way back to the box. Look in particularly at tie rod ends/pitman/idler. If no slop or play observed, look specifically at the input shaft vs the output movement of the steering box.
Oh, something else that I've seen on the older body styles (especially with taller tires) is that the frame will sometimes crack at the steering box mounts.