There is a 13 mm pinch bolt on the steering shaft at the steer rack. Remove the bolt and slide the shaft off of the rack. You should be able to check the U joints in the steering shaft now. I think you will find a stiff joint in the shaft.
Did you ever find the cause of this condition? I just drove my truck today after it sat for 2-3 weeks and it is doing the same thing. I was thinking it's probably one of the knuckles hanging up after all the salt and sand this winter, but I was curious what you found