Mitsuman;
Your truck doesn't fall in the group which required steer column replacement. Now as I understand you, the travel drive is functioning, and you have turned the steering wheel first to the right steering lock, then to the left steering lock, and held it briefly also with the engine running, brakes released and the FWR and/or the REV pedals depressed and you have turned the steering wheel and steering is not functioning with truck still driving, but when the truck is stopped steering is functioning. This sounds as if this truck's tandem pump being defective. To best confirm this, it is would be advise that a measurement of boost and maximum pressure test be run.