OEM nav units *can* be VIN locked and must be reprogrammed by the dealer. It would be cheaper and less frustrating to go aftermarket. A Sony unit with TomTom nav runs about $800 plus about $200 in additional parts and labor. Plus you'll be able to take it out if you ever get rid of it. Since I do not believe you have SYNC, you're not losing anything, but rather gaining a lot. You can amazon or eBay a radio with everything at less than $300, but you are asking for headaches.
However, if you are dead set on the OE navigation unit, make sure you have all the needed parts such as: amplifiers, subs, antennas, wiring. Some of the head units do not do well if it doesn't have all of it's pre-programmed parts and will throw errors at you.