The steering wheel is loose. I know this can be caused by a number of problems, I just need to figure out which one.
I would start with a vehicle inspection from a YourMechanic technician. Ensure that the technician inspects the intermediate shaft, steering gearbox, and all joints associated with the steering system. If any parts are found to have looseness or play, they will contribute to or directly cause your problem. The steering gearbox may also have an internal issue causing play in the steering wheel. The technician will be able to give you a correct diagnosis of your problem and be able to repair it as well. Be sure to have this issue repaired as soon as possible in order to keep your truck safe.