You can not check steering wheel / steering box center with draglink hooked up. The steering stops could have been messed with, or you can have bent or worn parts that throw off the adjustment. If the wheel is not centered while checking just the gear box, then moving the steering wheel on the splined shaft is the way to go.
With out more information on how the truck drove prior to spring changes, one has to assume that the spring change made the difference or the owner wouldn't be asking for suggestions, thus the location of the axle has slightly changed and moving it forward or backwards should correct the problem. We will have to wait for more information.
