G'day rtv, I was going to reply before but I thought I'd see how the inspection went. You haven't been locking the diff on bitumen or anything I assume? Have you tried another Ford dealer, some of them know more than others & it would be worth a go.
Also try on www.uteman.com
, there is a topic on the RTV ute where I remember there being a different problem with the diff a while ago. There is a user on there called anakie, he is one of the Ford ute engineers, he should be able to give you some help.
Good luck with it.