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4wd is used to send power to the front and rear axle of the vehicle (as oposed to the rear only when it's in 2wd). Once the power reaches the axle, it is up to the differential to send the power outward to the tires... on an axle with an open differential the power is sent to the tire with the least resistance (so, if you have a tire spinning, the differential will send even more power there), on a limited slip differential power is sent to both wheels.
Shifting into 4-low will only changes the reduction ratio in the transfer case, it does not 'lock in' in aditional wheels or anything. 4-low is used to provide a really low gear ratio for slow speed, or for the added torque needed to climb a steep grade and such.
My guess is that your F250 has open differentials (most trucks do), so once the power is sent to the axles the differential sends it to the wheel that is already spinning. If you are not getting any power to the front wheel(s), I would guess that your hubs are not locked in (if they are manual locking) or have a vaccum leak (if they are automatic locking).
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