I have a 2003 F-150 with 4-wheel drive... sometimes it does anyway. it seems to kick out on its own even though the sensor says its engaged. it seems to stay in if i'm driving downhill but kick out if going uphill. any ideas?? Thanks!!

 

Don't have a 4X4 truck myself, but I would say it's a solenoid or connection issue (if it uses a solenoid to engage) or a vacuum hose issue (if it uses a vacuum like my Jeep did).
I doubt it's serious if the 4X4 works most of the time.

 

Next time your going up hill in 4x4 and it kicks out have someone look underneath and see if the front driveshaft still spins. If it does its probably something to do with thw vaccuum in the front end. If not then it has something to do with the transfer case.