Wow this thread took a left turn, but since its heading this way I'll add my 2 cents. The crazy 15,000 mi. service interval only pertains to Dodge/Ram trucks as I have been driving Fords for quite a while and the only remark in the owners manual for servicing the rear end is if it has been submersed in water. On another forum I frequent a retired Ford engineer stated that they regularly saw 304f on their testing at Eisenhower pass and went on to say don't worry about it. For a short time Ford Super Duty were coming factory equipped with an aluminum rear diff cover ('08-'11) and it was decided that it was more bling than function as overheating a rear end isn't possible per their own engineers.