96 Ranger 4x4 problem

Automatic hubs. When driving in 4-High, minimal problems - occasional clunk. After disengaging, the driver side will sound like it re-engages. A humming, winding sound. Ill stop, engage and disengage. (If I dont do this, the noise gets pretty bad.) If I dont hear that sound, there will be a repetitious clunk as if it is still partially engaged. Tried out 4-Low, works fine. Any idea on troubleshooting?