Diesels are the only way to go here in Australia. I used to drive a Nissan Patrol which is one of the more popular models here. If you are in the outback and gas stations are 650+ KM (400 Miles) apart and there is a chance they only carry diesel, you want to be running on it.
I can see why you call these trucks, they are probably great for carrying or pulling seriously big loads but dont look like the ideal off raod machines. Ground clearence looks to be an issue with those long wheel bases. For that I am thinking, Jeep, or Humber? Would they be the top brands in the US for that purpose.