I'd second the notion of seeking out a complete rear end for economy's sake. Gears alone are only a part of the price. Sometimes you end up replacing bearings and should usually replace seals. Professional set-up is a must, as the pinion depth, backlash, mesh pattern, etc have to be right or it will get noisy fast and could wear prematurely.
Remember with gearing, the higher numerically you go, the lower the top end speed, as for any given rpm, the vehicle does not move as far. Usually this results in better acceleration, at the expense of cruising mpg. The reverse is true, too, so lower numerical gearing generally works the other way, but for both "rules" there's more at play than appears on paper.
John B