That's bad luck.
One thought I have whenever I see a diff leaking is how well did the previous owner/dealer do at keeping the diff filled. A differential is easy to neglect and get run dry, and a dry diff won't last long. As part of your pre-purchase visual inspections, did you check the diff fluid level? After you bought it, did you check the diff fluid level occasionally? Does the warranty expect you to pay for a diff rebuild, or will that be covered, too?
One thought I have whenever I see a diff leaking is how well did the previous owner/dealer do at keeping the diff filled. A differential is easy to neglect and get run dry, and a dry diff won't last long. As part of your pre-purchase visual inspections, did you check the diff fluid level? After you bought it, did you check the diff fluid level occasionally? Does the warranty expect you to pay for a diff rebuild, or will that be covered, too?