I noticed when I drive my '96 GT usually in 4th gear when I accelerate I hear this loud ass noise coming from the back. Well, it was about time to take my car into servicing and the guy asked me if there were any problems and I just mentioned that. Now my car has been in there for like 3 weeks and they say it's the differential gears and they have to "re-build" the entire rear end. What is the deal with this and is it a severe problem?