1990 Dodge Ram Knocking/ Fluid Loss

Transmission problem
1990 Dodge Ram V8 Four Wheel Drive Automatic Unknow miles

I recently purchased a 1990 Dodge Ram W250 with unknown mileage. After changing the oil and topping off all fluids I drove it 20 miles with no problems. On the return trip however, it suddenly began to knock and rattle. Once home I left it running while atempting to locate the source of the noise. Upon looking under the truck I found fluid leaking to the ground and smoke coming from under the hood. I turned the truck off right away and opened the hood to find fluid sprayed on everything. The smoke was coming from where it hit the hot motor. I checked the oil (the leaking fluid was brown), but the oil was still full. I also checked the transmission fluid and found it almost empty. Any idea what might cause sudden loss of transmission fluid, a rough idle, and lots of knocking? Thanks!