'04 Ranger has "live axle", which means front wheels, front axles, and front drive shaft are always connected, if wheels are turning so is the drive shaft.
The rear "u-joint" on the front drive shaft is a common failure, but tends to make a popping noise any time you are at low speed, whether in 4WD or not.
Google: Explorer Propeller Joint
There is a video of how to rebuild this joint.
If it is only in 4WD and only when turning then could be one of the front wheel CV joints, but again these are always "in gear" just not powered in 2WD
Last edited by RonD; 11-15-2015 at 08:36 AM.