Rear differential broken

After long drives (hour or more) when I would slow down and turn there was a loud noise that felt like it was coming from the wheel bed and the car kinda shook front to back. I had a mechanic look it and he said (from driving it only) that the rear differential was wearing out and when it got hot from long drives it was the cause of the noise/shaking. My boyfriend was driving it and heard a very loud noise then nothing and I think that it finally broke. I tried searching for parts because my mechanic told me a new part would be $1000. I'd like to find a used Subaru part but rear differential doesnt have anything come up. Is it just a gear i'm looking for or is it lots of parts to make up the differential?