I have a 2003 Ford F150 with the FX4 package and 112,000 miles. My 4x4 is electronic with the turn of a ****. I can't quite pinpoint the problem so I brought it here to seek some help. When I put it in 4x4 the light comes on but the front tires don't spin. When it is on jack stands and I put it in 4x4 the tires spin no problem. Strange I know but, Does anyone have any suggestions on what the problem could be.

 

When the dash light comes on it suggest the issue is right at the front axle.
On the axle is a vacuum actuator. This device moves a slide collar in the front axle housing to couple togather the front CV shafts.
Check for this action by having someone switch the 4H in and out to observe it.
If the actuator does not move, back up to the lines feeding it and one solenoid on the fire wall.
The two vac lines; one pulls the diaphram one way the other pulls it back as determined buy the path through firewall solenoid .
Somewhere in this line up you should see the malfunction occur..
It could be the lines ,the actuator, the solenoid or the link drive to the collar.
Good luck.

 

Thanks for the help ill give that a try as soon as I get time.

 

What really was throwing me for a loop was that it wouldn't work on ground but when I put it in air it would work

 

I'm not sure how the coupling slides itself together in the front axle assembly. But since it can be engaged with the truck moving, my guess it slides together with some type of taper. If the solenoid on the firewall is weak, or say a cut in the vacuum line, or a gunked up fork...maybe it doesn't engage fully...thus letting you get "some" lock in if not under any type of load.....,just thinking out loud here.

 

my truck did the same thing. it turned out the Vac lines got into the air compresser pully and it did not cut them, just nicked them, I could put it in 4wd then in nuteral and rev it up then it would go into 4wd and work fine. Start by checking the 2 Vac lines to the frount diff. i think one is pink and one is blue. If nether of them are dammaged, take the actuator off and clean it, then try again. The reason it will work on jack stands, is becouse it has no pressure (it is freely turning) i bet it is vac lins or the acuator.