with a high rear I don't think you would gain anything by going to a larger tire. I have 4.10 rear w stock 235/85/16 tires and run 2100 @65 and goes up fast from there fuel mileage plummets. on my 99 285/75/16s helped to knock rpm back, had 4.10 rear. But with bigger tires you will wear out your unit hubs prematurely, ask me how I know!
I avg around 15 mpg with stock tires and low rears, everyone I know that has the 3.55 or 3.73 in the 3rd gens gets upper teens.