4x4 not disengaged, making whining sound
I have a 05' Ford Ranger Edge, auto transmission. I have been using the 4x4 a lot here recently, and it has worked great up until a couple of days ago when I noticed it started making a whining sound when I engaged the 4x4. The 4x4 light on the dash lights up and the 4x4 does work good, but when I disengaged the 4x4 the light goes out but the truck remains in 4 wheel drive for miles, and then all of a sudden something pops real loud and it's back in 4x2 mode, it's done this popping trick twice now, it sounds real bad and sends a jolt through the truck that I can feel. Any ideas of what's going on with it. It's been a great truck, has 43k miles.