Turbo2000-Lee

After owning my 2000MY Impreza Turbo for 5 months i started getting a nasty clonking through the drivetrain, so when i got it serviced a couple of weeks ago i asked the garage to investigate. They have told me the centre diff is on the way out. I have ordered a new centre diff and was wondering whether anyone out there had any advice on changeing one at home? For example does the centre diff share the gearbox oil or is seperate? Will i need a gasket for where it meets the gearbox? Is it a difficult operation in general?

Thanks