I have a 1998 Ford 4x4 Expedition. It has 120000 miles on it. Recently took it for it's yearly emission test, and was told there is a grinding/rubbing metal sound coming from the back of the truck. They said it could be the rear differential, wheel bearings or brake pads? I have not personally heard any whining or strange sounds. Should I bring it into the shop and how much would something like that cost to repair? Thanks!