It's either alignment or a sticking brake caliper. You can check the toe by measuring from the inner sidewall of one tire to the inner of the other, both in front of the axle and behind the axle. The front should be approximately 3/16 of an inch closer than than the rear. Next, find the exact centerline of the truck, front to back and mark it. Then measure from sidewall to centerline on both wheels. The measurements should be within 1/32 of a inch.
To fix front toe, loosen the adjusting collar between the inner and outer tie rod ends and turn it, keeping an eye on which direction the wheel is being forced. Once the desired reading is obtained, tighten the nuts on the adjuster collar. They are 1/2 inch.