jack up one rear tire, put the trans/tcase in neutral, rotate the raised tire around twice while counting the times the driveshaft rotates. about 3 times is 3.07, 3 and a half times is 3.55, etc. make a mark on the driveshaft to make it easier. if the rear axle is a posi, then you'll need to jack up both rear tires and rotate the tires around once.