I have 2003 F150 S'crew 4X4 XLT. I just bought it and havent had it in 4 wheel drive yet until this weekend. I was on sand and engaged the 4 wheel high. THe light on the dash turned on indicating 4 wheel drive. However, upon accelerating, only the rear wheels dug in and spun. The front wheels never began turning. I then thought I would try the 4 wheel low range and put trans into neutral and turned switch to 4 wheel low. I heard the clicking noises and saw the indicator light up with low range. I again tried to go and again, only my rear wheels spun. Wierd. I then checked to see if both drivelines were turning while truck was in 4 wheel while my wife drove slowly and they were both turning. Has anyone else experienced this?

 

You may have a vacuum leak. The front diff is actuated by vacuum. At the firewall, passenger side are 2 solenoids which actuate the vacuum to lock/unlock the front diff. If the one to lock the front diff is bad or the line from the solenoid to front diff is broke, the, you do not get the front axle locked in. Obviously the front shaft is turning, so it is not a tranfer case issue.
The other possibility is the front diff vacuum motor is bad.
This may all still be under warranty, depending upon milage. Under 36k?

 

Yes, the truck only has 19k on it so it is still under warranty. I will check out the vacuum and motor issues. Thanks!!