Last time I commented on a similar thread I pissed a bunch of people off for some reason, but...
We can only guess. It could be any number of things (ball joints, tie rod ends, wheel bearings, control arm bushings, steering rack mounts, loose lugnuts etc). You could try to wiggle any parts around under the front end, but while it is sitting stationary with weight on it, it probably won't do any good.
Put the truck on a rack (or on jack stands with the weight off of the suspension) and try to move the wheel/tires around, and simply look to see where any play may be coming from.
2006 F150 FX4 Super Crew - Bilstien 5100s leveled 2", 35" KM2s, 4.56 gears, Detroit locker, Eaton E-Locker, Traction Bars, AEM intake, Xcal 2
2010 VW Golf TDI 6MT