1988 Chevy Silverado Had Trans rebuilt less then 2 years ago

I had my trans rebuilt a couple of years ago. I was driving the other day and it felt like my tires were slipping on ice when I turned the corner. I let of the gas and then waited a second and pressed the peddle down again, but the engine only reved. I pulled over thinking I had bumbed it into (N), but no luck. The engine will rev and when I get to 4000 RPM or so the truck will begin to creep foward when in gear of creep in Reverse when in Reverse. I had checked my fluid about a month ago and it was fine. I checked my fluid the next day after this happened and it was a dark grey, about the color of pencil lead. I am not much of a mechanic, and could use some help here. Thanks