What sort of a Defender do you have? Is it a Salisbury on the back or a Land Rover one. If its like the front ones they do vary in size depending on the model.
What was the part number? I replaced mine last week, my 110 started life as a v8, now a 200tdi. I didn't have a clue what to get, measured face to face and went on lrseries website and they put the lengths in the description. Simples.