I have a '99 Expedition EB with 230,000 miles. Yesterday when I was accelerating to merge onto the interstate, there was a loud banging coming from underneath the vehicle. It stopped when I got up to speed, and it didn't happen again until I was accelerating onto the interstate to return home. I crawled under the vehicle this morning and noticed a coating of grease from where the drive shaft enters the rear end, to the differential cover. It was black and thick, and appears to have been there for a while, but there is nothing on my garage floor. Could this leak be related to the banging? Is this going to cost big bucks to repair? Thanks for any advice!