2000 Expedition Front Axle locked!

My front axle is acting as though it is locked in 4WD-LOW. When on pavement and making a turn, the tires grip and the outside front wheel skips to catch up. The front axle will turn independent of the transfer case drive shaft when the vehicle is in neutral and up on jack-stands, but the front two wheels will not turn independent of each other. For comparison, I checked the operation of the rear wheels and found that they will turn independent of each other while in neutral and up on jack-stands. Is this a transfercase shift motor/sensor problem or front axle problem. Thanks in advance. :confused: