I have a 99 f150 4.6l and when I pullthe shift lever into 4wd (high or low) the indicator light comes the transmission switches but the automatic hubs dont lock in. occasionally I will hear a grinding noise like they are trying to but it will go away after I let off the gas. In desperate need of help, hunting season starts on monday and I dont have 4wd.

 

I worked on an explorer that had an electricmotor on the four wheel transfer case to shift it. I replaced the motor (tested bad) and everything was A OK.

 

You may want to check for a loose wire or bad harness plug to the motor too (assuming it has one). I mostly usually work on older stuff so unless I rifle through a book I can be off a bit, but I put one in a few years back and I believe they're like Explorer but a touch bigger. The Explorer had the 4.0 motor and automatic transmission (which I installed as a used from a Mazda Navajo which is the Explorer clone).