It is possible that its not the N23 issue, if your transfer case motor has the fault the update was designed to detect it will give the same symptoms. Is yours a QuadraDrive II? If a Diff solenoid fails or the wiring to the solenoids is damaged then you get the same message on the dash.
You are to far away from me otherwise I would scan it for you
I recommend you get yourself a scan tool, then you wont be at the mercy of the dealer and there $$$.
Look up BlueDriver, it has good reviews and guy's here & in the states use it on their Grands. It will cost about the same as 1 hours labour at the dealer.