Nissan Navara: Differential collapse

I have noted with interest the reports of problems with the Nissan Navara. The differential in my son's 2008 Nissan Navara D22 Dual-Cab collapsed after just 30,000 km. The dealer said the problem was caused by shock overload and it was therefore not covered by warranty. I found it strange, as the vehicle is a four-wheel drive yet only used for light duties in the suburbs of Brisbane carrying toolboxes and ladders and not used for any four-wheel driving. As it was needed for work replacement original Nissan parts where obtained and the differential repaired. The company that carried out the repairs found on dismantling the differential that the problem was caused by a loose bearing that was not fitted correctly and had worked loose. This could only have been found by dismantling the differential and not just looking through an inspection plate as the dealer had done. A letter to Nissan in October 2009 resulted in no answer. How do you bring an international company to account when they will not even reply to a letter? It would appear that they are a law unto themselves. Your advice on a direction to follow would be appreciated.