My truck spewed out all of the fluid and wont move when in drive....what happened?

We just filled up the transmission fluid last week, didnt overfill. Then I was on my way to work and it was pretty cold out, all of a sudden while on the highway, it felt like I was losing momentum, so I gave it gas and realized even though the engine was still running, I wasn't able to accelerate. Seemed like I was in neutral. So I pulled over, stopped, put it in park, checked each gear, then it took off for a few feet, then wouldn't go again. So I pulled way off the road, and popped the hood, nothing wrong under there, then I noticed what looked like transmision fluid steadily streaming from I think the back of the engine all the way to the transfer case. We checked the dipstick, and it was just about all the way gone. It hasn't leaked at all when we filled it up earlier. But now I don't know if I need a new transmission (very spendy) or if its the torque converter or whatever that's called. It's a 1987 Chevy Silverado half ton four wheel drive..... somebody help!