I have a 2005 f150 with the electronic 4x4 switch for the 4h and 4l. At a sit still, the 4x4 will not lock in. Going at very low speeds, it will lock in but sometimes it pops back into 2 wheel drive after some distance. At speeds around 10-15mph, I turn it into 4x4 and can hear it click over but then hear a grinding noise coming from the front. Turn it back into 2 wheel drive, the sound goes away. Just yesturday I attempted to pull a car from a ditch, I was in 4x4 at the time of putting tension on the tow rope and as soon as I started pulling, it popped out. Any suggestions?