Chris is right on the money, you need to know center of gear box before moving the steering wheel. Somewhere around 2 turns each direction from center. Might as well adjust the gear box at the same time. If you find the center doesn't tighten up, then you have worn parts.
Check all this with the pitman arm unhooked from the steering linkage.
Get wheel and box centered correctly, hook up steering and go for a drive. Toe-in adjustment, worn or broken leaf spring center bolts, worn leaf spring main bushing, loose u-bolts, incorrect draglink, bent steering arm, and even different ride hight front and rear can cause problems.
If you absolutely cannot keep wheel centered, look up in the tech section of this site for information on building an adjustable length draglink.
The truck will drive much better with gear box straight with the wheel and when driving straight.
