Mate, sorry to hear of your troubles. Some dealers are the biggest wankers you will ever meet. I went thru all the same crap when i ordered mine, so i know how you feel. Apparently ordering a car from a factory requires a uni degree or something, cause they never get it right.
You could always convince them to install an air locker for you at thier cost. It would only cost Mitsu about an extra 1k on top of the $500- you spent for the factory diff lock. There is no way you should be made to wait until March, or even December, that is absolute bull..........good luck with it, play hard ball with them, dont back down.
BTW, your letter to MMAL will get a response along the lines of "your complaint has been noted, thankyou" they could not care less either.
Eventually, when you do have your new ute, you will be very impressed, they truelly are great trucks, just dont go to the dealer for any help, advice, servicing or parts.