I don't have any experiece with your specific truck as they don't sell them in North America.

However, in my experience most diff failures can be traced back to operator issues. Spinning the wheels, getting stuck, dropping the cluctch etc...

That being said the bills do seem very excessive.
If you give me a specific model and s/n, i'll put in a query at Shyster and see what i can dig up as far as grams etc.