Several people have had this problem, and Ford has not been able to come up with a good (cheap) fix yet. I've heard that is caused by a suspension problem, not brakes, tires or wheels. Apparently, when you brake hard, the front axle winds up slightly; when this occurs the wheels toe left and the truck goes left. Since it toes left, you shouldn't feel much in the steering wheel, right? If it were a brake, tire or wheel pull, you would feel the reaction in the steering wheel.

The soft spring bushings allow the front axle to wind up. They may have some stiffer bushings out by now, which may help.

There is also a brake pull that was caused by mis-matched front brake pads; I think there was a bulletin out on that. This could cause the brake pull either left or right, and you may feel it in the steering wheel. If your brake pads cause a left pull, and you have the axle wind up, then it will really pull left.

By the way, the NHTSA has a safety investigation on this subject; EA01-004. If your pull is real bad and Ford can't fix it, you might want to report it to them at: www.nhtsa.dot.gov
If its a real safety problem, they may be able to require Ford to recall and fix them.