Usually when you have to adjust them that far to make the beam pattern right, the truck has been in some sort of small accident, bump, or fender bender in the past... A small tweak to the radiator support is all it takes to throw the headlights out of alignment. My father's Sonoma had the same issue. I finally got the beam patterns to even out and point strait, but one headlight just looked way off, didn't line up with the hood or grill very well at all. One day when I was under it I noticed the bumper support bar bent and could tell that someone had loosened all the bolts and wiggled things back into alignment so the damages were less noticeable from the outside...

Another thing that'll cause it is aftermarket headlights that aren't quite made to OE size specs. I have clear lens headlights on my 04 and they are both slightly out of whack compared to the grill and hood, but not too terribly that it bother's me. The beam pattern is level and straight where they're at so I live with it lol

That is the major downside to the headlight assemblies in the S Series body style, the fact that the entire headlight assembly has to be moved and manipulated to change the aim, instead of just moving and adjusting the reflector housing inside the light like most other vehicles... Up side to this is replacement headlight assemblies are less expensive lol

 

You have four tabs that are held in place by the slides. There is also a tab slot in front that is sometimes difficult to get into and could be causing your alignment issue.