"dude"...I'm not that young.
Easier to just jack up the rear and spin the tire.
Mark the driveshaft with a marker/chalk...mark the tire .
Jack a tire off the ground ...rotate tire 1 full rotation slowly and count how many times the driveshaft rotates.
3 1/2 times = 3.55's
3 3/4 times = 3.73's
4 ish times = 4.10's
4 1/2 times = 4.56's
and so on.
If you pull the cover you need to do a diff fluid change..if you have never done a diff fluid change since your purchase of the jeep. It is a great idea to do.