For you non-technical people, this is the round thing in the middle of the back axle, and is where the rotation of the prop-shaft is transferred to the wheels in some mysterious way.
The procedure here is the same as for the gear box although to my amazement, the plug on my car this time requires a square socket ! That's two you'll have to buy. If you need to fill the differential, use SAE90EP gearbox oil and note that the different oils are not interchangeable. Capri diffs. are very rugged and as long as there is some oil in them, they should last a lifetime. All models take 2 pints of oil. See my work section on how to remove the cover and change the oil.