My 4WD Low sometimes engages on its own and is difficult to get out of it; no light changes on switch. Can't tell if actually engages or if the engine just reacts like it is engaged. Tachs about 500 higher and increased sound in engine compartment. Otherwise appears to work normal. WTF???

What engine/transmission do you have? How fast are you going when this happens?

It may be an overdrive problem. Your engine would really scream if it went into 4 low at speed.

It's a 4.7L and I don't know the tranny. It's a 5 speed with OD. Generally it kicks in at a stop but occasionally at low speed.

Sorry, I mistyped. It's 4WD high, not low!! My bad.

4wd high shouldn't change the engine RPM. You would notice it "binding" during turns though.

Can you describe the sound from the engine compartment?

It sounds just like it is in 4WD Lo, the high whine, but the tach only indicates about 500 or so higher than normal. Pickup is a bit sluggish, but I can obtain normal speeds at a slightly higher tach. Strange huh?? Could it be the mode sensor or I/O speed sensor?