The v6 thirds have bigger bearings, bigger fins on the side (to try and control housing flex, not cooling), the housing itself is a thicker material, and there are also ribs in and around the carrier bearings. To run a 36" tire, you don't need to upgrade to the larger diff., as long as you don't lay down the law with the skinny pedal, and alot of horsepower. I know several people that ran 4cyl. diffs with bigger and heavier tires than 36" Iroks, and had very good success with them.