Proper gearing is just as important as your decision to make the swap. If you are building your rig for optimum fuel mileage, you will need to run 3.07 gears. If you are looking for a high torque off-road vehicle, go with a numerically higher gear such as 4.11 or 4.56. If you are not sure what gearing your Jeep currently has you can call your local dealer and give them your VIN #, alternatively you can look for a metal hang tag on the pumpkin.
You can also check out this site for a full explanation: