Axle ratios must match on a 4x4 vehicle, or serious drivetrain damage may occur. Many manufacturers attach a metal tag with the axle ratio on it. An example would be a tag with 3.73 or 3 73 stamped on it. This means that the axle has a 3.73 ratio.
If no tag is available, and you have a GM vehicle, you may be able to take advantage of an option code sticker. This is a 2.5 x 3.5 inch sticker that is usually located in the glovebox or on the cardboard spare tire cover in the trunk. This sticker has GM codes in alphabetical order which relate to the options of the vehicle. The "G" codes tell us what ratio axle your vehicle has. Examples of GM ratio codes are GU4, GU6, GT4, etc.