Your car probably has something in the 2.73 to 2.41 range. This isn't good for what you want to do. A 3.73 with a 700R4 or 200 4R transmission would be good. There is no way that you are going to get better performance and keep the gas mileage without using an overdrive transmission.
Swapping the gears is going to require a 3 series differential. If you ahve a 2.56 or 2.41 gear then you have a 2 series and it will need to be swapped. I recommend using a Yukon Dura Grip and a set of Yukon gears and installation kit. This will get both wheels pulling so that you are not just spinning one wheel. The Dura Grip is not that expensive and it works well. There is a member on this sight that is a Yukon dealer. Prostreet6t9 can help you with the parts.
You must know how to set the pinion depth, bearing preload and backlash to set up a set of gears. You must be able to read the contact pattern so that you wil know what adjustments need to be made to the pinion depth and backlash. Changing gears is not a bolt in job. It requires shims and patients.