I had this question asked to me a few times. A lot of the time the metal tag is missing from the differential cover or you aren't able to read it because it has been attacked by rust or is just covered under years of grime.
If this is the case, the gear ratio for the rear end is also stamped on the passengers side axle tube. This is for both XJ Cherokees and MJ Comanches. Just remember that XJ D44s will be SOA while MJ D44s will be SUA from the factory.
MJ: 3.07 Gear Ratio = 605325-12 3.54 Gear Ratio = 605325-10
XJ: 3.07 Gear Ratio = 605342-1 3.54 Gear Ratio = 605342-3