Expeditions have independent rear suspensions and F-150s have solid rear axles with relatively flat ends on the axle shafts. The Expedition rear hub assemblies have the long spindle shaft probably due to some machining requirement for that extra length. If not, some bean counter would calculate the fraction of a cent savings that could be achieved by eliminating that extra half inch or so of shaft plus machining the extra five or six threads. In practice, the threaded shaft beyond the spindle nut is surplus so you can cut off enough of it to allow for the F-150 center caps to fit without compromising shaft integrity. Usually less than an inch to remove, then clean up the cut end to with a file to allow easy removal and reinstallation of the spindle nut should it require service sometime in the future.