well for the sake of longevity an entire axle housing would be the best route. and besides its not that hard. if you replaced the carrier you would have to check your pinion depth, backlash and gear tooth pattern and adjust accordingly. trust me, replacing the entire axle assembly is a lot easier. and im pretty sure you can change the ratio thru the computer, just tell it its a 3.73 but you have to have a scantool that can do it. as for gas mileage i get about 26-30mpg. i do a mix of driving too and i only beat it when i have to but i take care of my truck too

but when i do a long haul i run about 75mph for about 3 hours and get 28ish mpg.