have you recalibrated your spedometer/odometer for the larger tires? if not then you're getting better mpg than you think. based on factory tire of 265/75/16 circumference of 100" roughly and the tires in your sig with a circumference of 116" you're traveling 16% farther than the Odometer thinks you are consuming the fuel you actually measured (14.5 - 15.3).
Better mileage is better mileage regardless of where you start from so let's figure out how to help your mileage, but don't leave out return on investment (ROI) time.
Factory gears are set for a compromise between towing capacity and fuel economy and acceleration. If you're driving highway miles then I would consider looking @ 4.10 gears which were available as a factory option so salvage yard pieces are an option heck you can get a rear and just keep it out of 4wd and test it for a week or two. If you like it then you're in less than R&P, setup kit, & setup. With the 4.10 you will keep the longer legs on the highway with reasonable acceleration. If you're not towing then keeping the OE gearing is not the most fuel efficient erring on the side of slightly taller (numerically lower) gear tends to yield better cruising economy. I can say that in my switch from 33 to 35's with my 4.10's yielded slightly better economy calcs with a non adjusted odometer which are even better when I consider the tire diameter calculation. Given the air you're displacing 15-17 @70-75 is about where you're going to peak.
Given the mileage you're driving you may want to evaluate the costs of a commuter car if you do not need the capabilities of the PSD. I made the leap 7 years ago and couldn't be happier. It makes sense and ¢ for me and I'm currently only driving 300ish miles/wk. You can search for other posts I've made regarding upgrades for the purpose of mileage and showing the extremely long ROI time making it a money loosing proposition to change the status quo.
