Principles of Operation: Ratcheting
WARNING: This page is about a different car, the 2005 Ford Pickup, 2005 Ford F550 Super Duty, 2005 Ford F450 Super Duty, and 2005 Ford Cab & Chassis. However, it is still accessible from the selected car via links, so may be relevant.
If a "ratcheting" noise is found especially over road bumps and potholes, suspect the needle bearing that supports the inboard end of the axle constant shaft within the hub/bearing assembly. Eliminate this as a cause before servicing the hublock. Wear at this bearing can set up vibration, which may be heard as ratcheting, while a seized bearing can bind the shaft, forcing it to rotate in 4x2.