Cuscos make aftermarket plated diffs for Jap cars.
Lots of versions of various harshness's and operations.
Do you know any more details on it?
Miles better than the std one IMO, tho some people are either scared of th way it makes the car behave, or dont like the noises it makes, and sells em.
Depends what kind of person you are and how you drive, not to mention what diff it is.