2003 Lincoln Navigator will not shift from all 4wheel drive into 2wheel drive

See that this is an old problem. Some of the knobs could crack on the backside and allow the knob to rotate on the shaft without turning the shaft at all. If the knob is okay then go underneath the vehicle and disconnect the electric plug on the shift solenoid and Inspect for corrosion on the terminals. There is a problem which can happen if an indicator light burns out in the electric shift models. The length of the shift solenoid plunger is what determines whether you are in 2 wheel or 4 wheel drive, or high or low range. If the indicator light burns out, it changes the electric power and position of the shift solenoid plunger.

Does it shift to 4wd high range ? If so then shift to 2wd . What is going on here is the transfer case motor is stopping due to the motor position sensor has gone bad which means that you replace the transfer case motor assembly. But if you wish to make it a 2wd for awhile then remove motor and manually turn shaft . This can be done by raising and supporting rear axle truck in park remove motor turn shaft slowly with pliers and rotating the play in drive shaft. There are stamped markings on back of case showing gear positions..

the swicher have problem,check to expert mechanic

Can you supply more information? year and model of vehicle, does it have an actual shifter lever or buttons? and what are all the available options to shift into?

If you have a honest mechanical manual 4x4 that you reach down to the floor and shift into 4x4 with a lever, and 4x4 doesnt work you got problems with your transfer case or you got some bent or missing linkages.

if you have a push button system for 4x4, then you could have a number of problems, bad actuator motor, bad wiring, etc.

You can "shift on the fly" at up to 45mph - just let off the gas and pull the shifter back to 4hi. DO NOT try going to 4lo while moving.

Another way is to stop, put it in neutral, and pull the shifter back to 4hi or 4lo.

This is vacuum activated - so if you can't get it to engage - the vacuum motor is failed - or you have a disconnected or rotted vacuum line.

Cherokees have the command - track. You'll see a lever next to the transmission shifter that has letters 2H, 4H, N, 4L on it.
You can shift from 2H (2wheel drive high range) to 4H (4wheel drive high range) at any legal speed.
If it's an automatic, I recommend just taking your feet off of both pedals, so there's not a lot of torque being passed thru the transfer case.
If it's a standard I recommend stepping on the clutch when you shift, but it's not necessary.
To go to Neutral just pull the shifter back one more position to the N position. To shift into 4L (4wheel drive lo range) you must be stopped.
I also recommend shifting the transmission into neutral when you do this. Push the shifter to the right, then pull it back.

Thanks!

does it have auto hubs, if so check the vacuum switch at tranny side

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The switch on the brake master cylinder is FAMOUS for shorting out. Unplug it and find the blown fuse. In the fuse box under the hood, fuse 1.5 Your truck may be under a recall for that switch, call the dealer with your VIN number and see if it qualifies.

Ford has 2 types of 4x4 system engagement. One is a knob on the dash that you just turn to 4wheel drive high or low. The other system is the 4x4 transfer case shift on the transmission tunnel (hump) between the driver and passenger seats. To shift into 4 wheel drive high while driving, just let off the gas and shift transfer case into 4H position, and continue driving. To shift into 4 wheel drive Low, stop the vehicle completely, put transmission in neutral, shift transfer case into 4Low position, put trans. in gear, and start driving. I am assuming before doing the above, you have locked in ( turned the front hubs to 4 wheel drive position.) Any further questions, let me know.

A safety feature on your car is that it will not shift out of "park" if you are not applying the brakes . Make sure your doing that while trying to shift out of park .