Re: 06 f250 no 4x4 low
Are there any dash lights on (check engine light/service lights)? If so, get them read. Most vehicle computers (PCM) will store any codes for 50-100 restarts so you can try scanning it and see if anything comes up.
Be sure to place the transmission in NEUTRAL before going into 4LO, as it shouldn't engage when in a forward or reverse gear. To put it back into 4HI or 2WD you'll have to go back to NEUTRAL. You can also check the wiring under the vehicle, to make sure that the wires are not broken, cracked, corroded or detached. I know in the newer F series trucks, the wires are fairly exposed to the elements.
Worst case, is you need to get a new TCCM, or transfer case control modele, which is considered to be a maintenance item with the electronically controlled transfer cases (knob, vs manual lever on floor which has no TCCM). I'd phone around local salvage yards for one, as a new unit from the dealer will set you back a good wad of dough. Usually, you'll get an engine code for this, but if not, I'd take it into a qualified shop (not necessarily a dealer) for a proper diagnosis.
'00 Durango R/T 360ci 290hp (modded); 138,500m
'06 Pontiac G6 GT 3.5L 220hp; 44,000m
'12 Chrysler 200 Limited 3.6L 283hp; 13,000m
'99 Taurus 3.0L 2V Vulcan 145hp; 154,300m - Traded
Amsoil in all vehicles!