Hey,, Loud rumbuling noise under my truck, i think is the rear differential.

What is your question?

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.