You jack up the rear, put the trans in neutral and turn a tire one rotation and count how many times the driveshaft turns. If you have a sure grip, it's pretty stratight forward.

For instance if you the driveshaft rotates 3.55 turns, you have a 3.55 to 1 ratio.

With an open rear end you need to have someone hold one of the tires and rotate the other one two revolutions while counting how many times the driveshfaft goes around. You can do this by yourself by just jacking up one wheel.

Here is a page with more information. http://www.jedi.com/obiwan/jeep/misc/gearratio.html