Broken rear springs/differential housing cracked at mount on 2004 Volvo XC70

Anyone seen this problem with their XC70? When changing over to summer wheels/tires, my mechanic noted a broken LR coil spring (some corrosion noted on the spring itself). Preparing to replace spring, he then noticed that the differential housing is cracked at the mount where it bolts up to the subframe.
Have taken car to dealer who reports only having seen a couple of broken springs before, and never this problem with the differential housing. This is a safety issue.
Note that Saab recently issued a service advisory and extended warranty coverage for a problem with premature breakage of front springs due to corrosion. I'm wondering if anyone else has seen this problem with their Volvo. By the way, you would not notice this unless you look for it.

Asked by for the 2004 Volvo XC70
Did you have the dealer check for any recalls or technical service bulletins for this issue?
Thanks for your answer - yes, but there are no TSBs or recalls on this issue. When I contacted Volvo they said they had never heard of this problem happening. This is a $5k repair using new parts. Since Volvo has no intention of fixing it (car has 89k miles and is out of warranty), I'm wondering whether replacing with a used differential is a good option.
Seems odd that your dealer says broken springs are unusual - my one front coil spring broke 2013 [one owner 2003 Volvo xc70, 30,000 miles]. When I googled problem, I found numerous reports by consumers and mechanics about xc70 springs breaking, even while driving and puncturing tire.