Wow, that's a fire! Sorry about the old rig, but the new one is really nice!
The first thing that will help will be to get a smaller tire. The least aggressive tread that will handle the weather is what to look for, or maybe a summer set, and a winter set. But those big and fat tires aren't helping you at all.
I too, have an Off-Road model. I notice that the mileage really suffers when you get to 75 mph and above......not that I ever drive over the speed limit mind you.....
. I'm not sure if your rural highway commute allows for those types of speeds, but that has been my experience. Also, try it with the O/D off if part of the drive is at slower varying speeds. If the O/D is hunting back and forth, it doesn't help MPG.
A better air filter set up, free flowing exhaust (including cat. converter), and full synthetic fluids in the engine, diffs, and tranny will help some too.
You are not far off the 15 MPG mark, and should be able to get there without major expense.