Greetings, new member here.

Basically, my car is making a loud howling sound above 40 mph and it appears I need a new rear differential. I was quoted by a local tech (not a dealership) for the part and labor $500. Someone on the MazdaWorld.org website said that was way too much. I have a friend who is really good with cars, (mainly Hondas) and has done a lot of work for me in the past, but he's never done a rear diff. Should I just go ahead and have this done? It seems like rear diff's are harder and harder to find and I'd like to have this fixed ASAP.