Loosen the u-bolts and use a pair of racket straps to pull the axle forwards, while being pulled, tighten the u-bolts back up. If it's even farther off center, pull the axle towards the rear of the truck. All you are doing is moving the axle in the alignment holes, it doesn't take much wiggling to get the steering wheel off center, the alignment holes are not very precise. Caster shims will also change the steering wheel center, did you install them with the thick end towards the rear of the truck? Most stock hight trucks use 2 degree shims.
