Have an 02 F150 with 5.4. I've been driving it daily for over a month now. Throughout this time, I noticed it had minor front end drift (if it catches a groove in the road, it will follow it). Yesterday, I pulled my boat with it for the first time. At about 45 mph, caught a groove in the road and was almost immediately in the next lane.
I purchased the truck a few months ago. It has what looks like 2-3" leveling kit on 295 Nitto's.
Today, I found a little play in the gear box. The previous owner stated he replaced "one" ball joint.
Could the play in the gear box be causing the sudden lane changes? Could it be the leveling kit? Or do I need to look deeper into the front end for other causes. A buddy was telling me that when I was turning the steering wheel side to side, without the truck running, he could hear noise from the gear box.

 

gear box is possible, but I would look at ball joints and tie rods first.. Jack under the A frames so the front tires are 2 inch off the ground, but springs and shocks are in the normal position, not extended. Put the edge of a 2 x 4 about 3 ft long under the edge of the tire and pry up and down while looking at the ball joints to see any clearance. grab the tire front and back and push- pull ... have your buddy look at the tie rod ends while you are doing this .... look at the steering arm and anchor also.. SLOP is probably outside the gear box.

 

After you check everything for play have front alignment done, sounds like too much toe out.. Or crank in a little toe in and see if this helps.