Yes, you can change the gears in the rear end. It is reccomended to have a knowledgeable person do it. It is probably about $400 to get new gears installed.
You may be able to find a complete axle at the wrecking yard that has better gears for mileage. That would cost less, but you don't know what you are getting. You would also need to know how to ID the different axles and gear ratios.