What do 4×4 locking hubs do?
4×4 locking hubs disengage the front wheels and reduce friction which may improve fuel economy by as much as one mile per gallon, depending on the vehicle.
The most common problem with hubs is that they won't engage. Mud, rust or worn parts inside the hub can prevent the mechanism from sliding in and locking the hub, as we talked about earlier. A hub may also fail to release for the same reasons that prevent the hub from engaging. Anything that binds the internal mechanism can prevent it from sliding and releasing. Look out for noticeable symptoms like an increase in fuel consumption, noise on the highway or increased front-tire wear.

Reference OE Number:

1C3Z-3B396-DA
1C3Z3B396DA
600-204
600204

Compatible Models:

2000-2005 Excursion 4WD;
1999-2004 F-series 250 Super Duty 4WD
1999-2004 F-series 350 Super Duty 4WD
1999-2003 F-series 450 Super Duty 4WD
1999-2003 F-series 550 Super Duty 4WD

Quick Heads-up for DIYers:
Disassembly and reassembly procedures do vary from one hub design to another, so refer to the OEM service information if you need help. If you don't have manual or online service information, disassemble one hub at a time. That way, if you can't remember how the parts came apart, you can always refer to the other hub as a guide. And a pair of nitrile gloves may be of help if you're disassembling and reassembling yourself.