[n_i_c_k]

I was driving my ranger when suddenly it stopped moving. It was shifting smoothly with no trouble at all. I got it towed home and checked the fluid in the tranny. It was black. There doesn't appear to be any metal filings in it and the transmission doesn't grind or anything when I put it in to gear. It's the 5R55E transmission with 4-wheel drive. Valve body? Transmission itself? Any ideas?
Thanks