Like Bounty said 2.73 are most common ( with the 25
but with the 360 3.54's are likely.... but unless you know the complete history of the jeep things can be changed easily..
best bet is jack up rear of jeep so tires are off the ground,
set axle down on jack stands
put jeep in neutral
place a chalk mark on the drive shaft and on the tire.
spin the tire one complete revolution,
count how many times the drive shaft spins.
3 and a half times and you probably have 3.54's.
not quite 3 spins and you have 2.72's
4 turns and you have 4.09's
or you can pop the diff covers like Tom said and look for the gear ratio marking stamp or count the teeth on the ring and pinion