There are a couple of things that could be going on. AD steering linkage is really primitive and your complaint is fairly common. I would start by checking the toe-in of the wheels, if it is off then you will have problems.
How are you determining that the wheels are pointed straight ahead? The wheels should actually appear to be slightly toed in if they are really pointed straight ahead.
The next most likely thing is the drag link- they are not adjustable, and wear in the drag link ends and spring shackles and eyes tends to move the axle location forward or backwards with respect to the steering box, changing the angle slightly. You can try replacing all the pivot points, but you likely won't fix it. There is a way to make an adjustable drag link (its in the tech tips).
The third possiblity is that your frame or rear axle is tweaked a bit and the truck is "crabbing" down the road. Have somebody follow you while you drive. If your truck looks like a bulldog walking, your rear wheels are swinging your truck enough that you have to steer to the right to keep going straight down the road.
