I am told the rear differential is worn and needs to be rebuilt? What is the part and how much? It absolutely must be done as there is a grinding noise and oil leaking. I initially thought it was back brakes.
rear differential rebuild on 2003 Jaguar XJ8
These types of repairs are hard to estimate before a tear down and inspection is performed. Sounds like you already have a shop to help out, if they have provided an estimate, post what work they are requesting and the price and someone may be able to say whether that's a good price or not.