You should really never run 4wd on pavement...period!...That is unless it's icy, snow-covered, etc., or if you don't TURN. A truck's 4wd set-up is different than that of a typical AWD set-up. An AWD is set up with a certain amount of "slip" in it to make turning safe on the transmission. A 4x4's differential is set up to NOT allow much slip in order to get more traction to the ground. When you turn in 2wd, the inside wheel turns fewer revolutions than the outside wheel; when you're in 4wd, they will turn the SAME amount of revolutions, therefore if there is no "slip" provided by the dirt (vs. pavement), that torque builds up and can cause a tremendous amount of damage. Try it at a very slow speed...put your truck in 4wd and drive straight on pavement...nothing. Then try to turn...it will be very hard and you will hear the tires "barking" trying to achieve some slip. For what it's worth, if your electric differential ever gets stuck in 4x4, back-up while turning (might have to try both directions)...sometimes torque can still get built up and it won't allow it to disengage until that tension is relieved. I've never seen it happen in a Ford, but I have seen it happen to a Chevy...my $0.02.