From your sig. you have an "02", so this should work for you.
On our model year trucks we can only adjust vertical, no horizantal adjustment. Anyway sounds like that's what you want.
Open the hood and look at the back of the headlight (more towards the the side closest to the front fenders) on each headlight. You should see a gray bolt with a washer that look like it has some teeth on it. That is your adjustment bolt for vertical adjustment. It's 8mm if I recall, move it to the right will lower the aim on the headlights. Hope that makes sense.