All of the above. Could be a number of things, but if you messed with anything recently that would be a good place to start.
Aftermarket steering links, or stock? Either way, make sure the adjusting sleeve clamps are not rotated in such a way that they could catch on something. Such as the lower trackbar bolt or double-shock lower mount, etc.
Ball joints could have failed, like brewchief was saying about the u-joints. Or they could both be on their way out.
Even wheel bearings or brakes dragging might not sound like they'd have similar symptoms, but if one is dragging it's putting some strange inputs into the steering. More often that feels like a "pull" one way or the other, but you never know.
And like said, of course it could be the box. If some debris is broken off from the inside, or the gears are worn out, or someone tightened up the preload adjuster on the top of the box, it could bind in the on-center position.
Sounds like it's time for a full inspection. Right from the steering shaft joint(s) on down to the ball joints, with everything else in-between.
Good luck. Let us know what it turns out to be.
Oh, and check the air pressure in your tires! With most modern truck tires, you could have a total flat and not even know it except for how it steers.
Paul
