Hey,, Loud rumbuling noise under my truck, i think is the rear differential. on 1998 Ford Expedition

My rear diffe. Was leaking but today while coming out the highway i heard a loud rumbling noise under the truck, when i press the gas on neutral theres no problem but when the truck starts moving it sounds like im dragging a piece of metal under. Can anyone tell me what the problem might be and whats like the price range to fix it? Thanks.

Asked by for the 1998 Ford Expedition
Did you add any lube or just run it dry? Special gear lube required!!!
2 more answers
did you run it dry.get an insp before anything else
Before you jump to to conclusions have the fluids cheked and vechicle cheked on a lift you may want to tow it to avoid any more damage fred hunts point auto should cost no more than fortynine dollars to check that