I doubt any Mazda dealer would do the labor for it.
Looking at the Mazdatrix website:
http://www.mazdatrix.com/G8LSD.HTM
It looks like they charge around $350 just for the discs.
Find a good drivetrain shop locally, and they can do the work for you.
Assuming the prices from Mazdatrix, a fully rebuilt diff is going to cost you around $500 - $700 for parts and labor.
-Ted