^It's not just new gears though, new diff too. Hopefully he didn't run too expensive fluids in the rear like $20/bottle M1 or something because good practice says you drain the diff at 500 miles of easy driving, check for shavings or any metals and then re-fill with clean fluid and go.
2000 Mustang GT Steeda #0048
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.