I'd say check your ball joints, but they typically get looser rather than tighter as time goes on. So instead, I'll suggest that you check your steering damper. More than one time I've seen a steering damper (once on a 4Runner and several other times on other cars) with just a very slightly bent shaft that will make the truck just a bit tougher than normal to steer. I'd be especially suspect of the damper if you have a slight returnability issue with your steering- ie, you have to help the wheels a little bit to straighten out after making a turn.
To test out your damper, just remove one side (or heck, remove the whole thing if you like) and feel the action- does it stick at all? If so, you've likely found your culprit. Good luck.