[Bluesman]

Take into consideration what gear ratio is suitable for your driving application. I prefer a 3.31:1. It is a very good hiway ratio with a 1:1 fifth or equivalent. Their should be an I.D. tag on one of the lower bolts that hold the back cover on the differential housing.
If the tag gives only the number of teeth on the ring and pinion...
no problemo. Divide the small number into the large number.
For example: 43/13 tag = 3.31. A very good article was written on the Jag rear end from 1960 to 1995. It can be found in an old Cobra, High performance Automotive Legends rag for 1995.