Check your front axle tracking arm bushings.
The tracking arm runs laterally, connecting one frame rail to the axle on the opposite side of the truck.
The factory rubber bushings wear out, allowing the ends of the arm to have play in the mounts, which allows the axle to move a little bit side to side.
Replace the factory rubber bushings with Energy Suspension or Prothane polyurethane bushings. The polyurethane bushings are much stiffer than the rubber ones and last much longer.
Other things that can cause the truck to wander are-
-Worn out tie rod ends
-Worn out steering gear box
-Worn out steering stabilizer shock
-Worn out king pins (ball joints on lesser trucks).
-Bad tire (tread seperation)