Does anyone know how to test the module off the car?

I would imagine it uses a canbus or some similar protocol to work so no I doubt you can test it off the car, fault codes should tell if it's working or not and they aren't repairable items as long as JLR are concerned so I doubt even they could help you out..!! Pete
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2014 L405 Autobiography SDV8 4.4 Loire Blue Ebony interior
2011 L322 Vogue SE 4.4 TDV8 Baltic Blue. Parchment over Navy Interior. Sold
2012 L322 Autobiography 5.0 Supercharged Ipanema Sand, Jet Interior. Sold
2002 L322 Vogue 4.4 V8 Epson Green, Ivory over Aspen Interior (Fatty Offroader) Sold
-Click for Project Fatty off roader-

I got the fault code P279D-02 four wheel drive range signal circuit general signal failure.

Not up to speed with the later cars yet, you could try someone like http://www.bba-reman.com/gb/index.aspx if it is a common failure they maybe able to help...  Pete
__________________________________________________
2014 L405 Autobiography SDV8 4.4 Loire Blue Ebony interior
2011 L322 Vogue SE 4.4 TDV8 Baltic Blue. Parchment over Navy Interior. Sold
2012 L322 Autobiography 5.0 Supercharged Ipanema Sand, Jet Interior. Sold
2002 L322 Vogue 4.4 V8 Epson Green, Ivory over Aspen Interior (Fatty Offroader) Sold
-Click for Project Fatty off roader-

I would get it tested in-situ using a diagnostic tool eg the IIDTool.

There can't be much wrong with it because it is a gateway on the canbus so if its electronics fail then all of the non-critical control units would have canbus communications faults.

Just got another diagnostic done. 2 different codes. P0666 and P181D

What is the diagnostic tool that is being used to read the codes? Important in the diagnosis is knowing which module posted the fault code.

P181D appears to relate to the TCCU calibration so perhaps a calibration is needed which might either fix the problem or reveal what is causing the original fault.

It was a snap on diagnostic tool. Is the calibration process easy to do?

It requires a diagnostic tool that is programmed to issue the command to tell the control unit to calibrate the transfer case motor. The motor controls the range changing and the centre diff clutch pressure.