Nice. Ford went to a 10 speed on their diesels and it really helped the mpg.10 speeds, not sure on the axles.
Wow, that's great mileage with that rear end. I know a lot of the F250's have gone to really small rear ends to get the mpg up. Usually the biggest is a 3:55, but a lot have the 3:31 (I think they even have a smaller one). My wife's Expedition has a 10 speed tranny with a 3:73 rear end (has the towing package) and the eco boost gas engine. It gets pretty good fuel mileage at around 19mpg around town, but nothing like your getting. That's awesome what you're getting with that rear end.My rear-end is the 3:73