[Otto]

Why not just use taller tires on the rear? This is much cheaper and easier than changing a tranny or differential, esp. if your existing unit is good, vs. unknown condition of a used replacement. TireRack et al have online data on tire circumference, traction, noise, longevity, etc.. I'm sure folks here also have anecdotal info on fuel economy of various tires. Easy to change if you don't like them.

And, as far as the fairings in front of the tires, use curved ones that are somewhat wider and taller, placed closer to the tires. A small plastic bucket of the approximate size and shape that Cool Whip or ice cream uses, cut longitudinally, would work fine. Reinforce with fiberglass, Great Stuff spray foam, or simply make a double layer. Spray it black.