whining diff - 2008 320i a/t

hi all, I need some advice...
my 2008 320i a/t with mileage 82000 has a terrible whining noise - no matter what speed i drive. i took the car to 2 independent workshops. they both pin the source on the diff. now both claim that no parts are available to do a repair. only option is to replace it at a cost of R28K
i just can't believe this - can the 320i E90 automatic's DIFF be repaired, or must it be replaced