I had the same issue... Thought it could be the gearbox.... Had it in for an inspection under warrantee, and it's the stepper motor(Globe motor) in the transfer box that required replacing... (Warantee item, fully covered and known issue)
Do a search on transfer box on here, or check the following:
http://www.mytouaregforum.com/forums/index.php?showtopic=430
Do a search on transfer box on here, or check the following:
http://www.mytouaregforum.com/forums/index.php?showtopic=430