AWD?
Not sure what differential that model has, but I can;t imagine it'll be a cheap replacement. 145k seems low miles for a diff to fail, so I'd want to diagnose further. Can you put it on a lift and run it? BTW, $520 for a DS replacement sounds high as well, but maybe that's a NY thing.
Coastdown diff noise usually means the lash is wrong (wear)... Honestly, I'd drive it until it fails, because unless you DIY it with a junkyard diff, it'll be expensive.