I'd tell them to send you an entire new rear suspension. On their dime. If they want the first one back, that's on their dime also. Document your request and notify ebay or craigslist or whoever. The diff may be fine but do you want to find out when your FFR is on the road and any recourse you have is gone because you bought it so long ago? More than just getting good parts you need, this kind of crap needs to be stopped.