Thanks for chiming in! I really appreciate it! I figured the diff was toast when I could spin one side or the other, and the opposite side didn't move at all.
I have no idea about warranty, and at this point, I don't really care. Because I won't be paying any shop... dealer nor private. I made that mistake ONCE in the past 20 years. Never again. You can't trust anybody to work on your stuff. That's why I do all my own repairs.
Yes, I bought the car second-hand. And (full disclosure) I bought it from an awfully shady dealer too. That being said, I don't believe they knowingly sold me a lemon. I took multiple test drives, and, had the vehicle inspected by a dealership prior to purchase.
The previous owner probably did neglect regular maintenance. None of the problems I've encountered are "abnormal" for a vehicle that's 10 years old and has 300,000km. But I also blame Ford's design for some of this stuff.
I've never really liked Fords, and I suppose since this is basically my first Ford, I should have done a lot more research before I pulled the trigger. The PTU failures seem to be a widespread and well documented issue, along with some of the other problems.
But, at the time of purchase, it seemed like a no-brainer. The price was great, the vehicle is powerful, fuel efficient, comfortable, and looks great. And it has the BEST heated seats I've EVER sat in.
Anyways. Rant over. Mystery solved. And I have a very good reputation with a local auto parts salvage yard. I will find a good condition/low mileage/used rear differential, and I'll replace it myself in my heated workshop at the farm.