1998 Plymouth Voyager doubtful diagnosis

I was traveling with my van when all of a sudden, it stopped moving forward. I was able to pull over to the side of the road. Upon inspection, tranny fluid was pouring out of the vehicle toward the front. When the tow vehicle got there, we poured 10 quarts tranny fluid into the van and drove it up onto the flatbed trailer. The fluid was pouring out, even with the engine off. The local transmission shop told me that my rear differential was shot- gears were stripped. They want to put a whole new tranny in- but I'm questioning whether or not it needs it? Any suggestions? Couldn't this also be a cracked pan, or something less drastic?