This might work...not the best method but can get close. put a mark on your drive shaft...and put a mark on your tire...put the Jeep 4th gear (1:1 ratio) ...push your jeep 1 revolution of the tire....while doing so, count the revolutions of your drive shaft.
I have not tried this myself ..but was told it would work.....
If it doesn't...forget I said anything!!!!!!!!!!