From your description it sounds like you are not getting any power steering assist from the power steering pump. You did not mention checking the level of the power steering fluid. I recommend you do that as your first step. Because the power steering pump delivers a lot of oil and the reservoir is small, it can be difficult to bleed the air out of the system if certain steps are not taken.
Normally when a steering gearbox is replaced, all of the fluid is lost. After replacing the steering box you should fill the power steering reservoir. With the front tires off the ground (truck on jack stands) and without the engine running, turn the steering wheel one revolution in a direction and stop. Check the fluid level - it should be dropping. Refill as necessary and continue to steer in the same direction until the steering stop is reached. Recheck and fill reservoir each time. Starting with the steering wheel in the center position repeat the previous steps while turning the steering wheel in the opposite direction. This should remove the bulk of the air from the system.
With the tires still off of the ground start the engine and slowly turn the steering wheel from one stop to the other. Shut the engine off and check / fill steering reservoir as needed. Your steering should operate normally now. In time all of the entrained air will be removed just by operating the truck, so you may have to top off the reservoir again.
Hope this helps.
- John