Get under the car with a light and mirror.
You need to see the ID tags and casting stamps
found on the differential.
Metal ID tags will be attached to a carrier cover bolt.
The one pictured has the numbers 43 & 13.
43 denotes the ring gear teeth.
13 denotes teeth on pinion.
43/13 = 3.31 differential gear ratio.
46/13 = 3.54 differential gear ratio.
If you have 43/13 tag, a drain AND fill plug, there is a
strong possibility it's from a ('73-79) Series II XJ6
The year will be stamped into the casting such as
shown in the attached picture of a '74 XJ6.
Moss Motors has exploded views of the entire
Jaguar rear suspension. Click the number on the
diagram and it takes you to the corresponding part.
Here's a link to the Moss Motors XJ6 suspension diagrams to get you started.
https://mossmotors.com/jaguar-xj6-xj...48506e2e8e326f