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Is it possible to steal a '01 Expedition without the key
Yes. Tow truck would work almost every time.
does this vehicle have a electronic validation between the car and the ignition key?
Yes.
If it is still possible to steal the vehicle, any tips on how to install a kill switch?
Pick a suitable circuit (fuel pump, ignition, coils, etc) and install the switch in that circuit someplace where you can reach it without it being too obvious. This will deter drive-away theft but will offer no defense against a tow truck.
If this vehicle has the electronic validation between the car and the ignition key, will it still be possible to start (& steal) this vehicle without the key?
does not matter what you do to if for theft protection if they want it they want it and it will be gone. Even if you install a kill switch they will find it in seconds