Dave, if you know it's knackered and needs replacing you can buy Molyslip one-shot treatment which will shut it up for a while. But make sure you buy the one for diffs !!!
If you have no joy with above mention diff then in a couple of weeks my perfectly good rear axle (drum brakes) is coming off to make way for a disc brake axle, so mine will be available if it's suitable (don't ask me I dunno)
Give us a shout if you want it.
Cheers