So what kind of metal shavings are you finding? Because it is 110% normal to find a bunch of find metal dust stuck to the fill plug magnet. As common and older than Jesus was born with Ford rear differentials and Dana axles. It's the nature of the beast with ring and pinion gears.
So to Update people on what happened. One local shop diagnosed the problem and I had another fix it (the first local shop didn't feel comfortable with the repair). I had googled around and learned that little metal shaving in the fluid was normal. But I had the diff fluid replaced at the shop that diagnosed the problem a few months before. When I asked if the amount of metal was normal, they said for how young the fluid was that there was far too much in there. Plus the grinding noise I had in the rear every time it sent power to the rear end added up to something being wrong.
When I took it to the shop that replaced the diff they put a camera up inside, and saw some teeth were ground down or missing. After that, they called me, confirmed the problem, and found another sport at a wrecker yard that had the rear end intact, and installed that one (saved me $700 on the new part).
From everything I gathered, from the 4 shops I called around to, googling, and searching this forum, I am incredibly unlucky lol. These parts on the car just don't break that often. The only thing the shops could say was that it must have been a bad unit from that factory that didn't break until my car was at 98k miles. That's the only reason I felt comfortable buying the used one. That car had 79k on it, but the shop inspected the diff before it went in and said it was good and once again, from what I gathered, the diff is one of the last things to go on a fully stock car.
Hope my $1,200 pain helps someone who might be in a similar spot. Now I get to have my brakes changed cause they blew a line in the parking lot when I started the car, but that one is more of my own fault. Was planning on doing a brake and rotor change when the diff broke. Then I didn't have the money after paying for the diff so I was waiting till June but alas they didn't hold up. Ohhh adulthood.
