My 96 Bronco dumped transmission fluid today after overheating

What is your question?

Was trying to pull a friends chevy out of some severe rutted mud environment today and all of a sudden a huge amount of smoke started coming from under the truck....briefly....I thought it was a tire smoking as the front of the truck was on pavement.. It didn't keep me from continuing to drive on up on the road and continued to pull him out with another truck helping by pulling me too. Afterward I looked back to where I was stopped and it started smoking and look like about a half a gallon of transmission fluid had dumped below the truck. Checking under the hood...the was no sign of leakage but no fluid showed on the stick. Looking under the truck show signs of a small amount of leakage where I had parked....coming from a circular seal of some sort under the truck. I can't find it on any schematic. My friend then went to buy fluid....took about 20 minutes....I never turned off the engine.....we added about half a quart first....saw no leakage...so almost 1 1/2 quarts more to to show full n the stick. Drove about a mile at between 20 and 30 mph...stopped...saw no leakage....hit the highway at 60 mph for abut 4 miles and parked it. The circular plug looking thing is about the size of a half dollar...appears to have a rubber oring or gasket above the metal....can spin with my finger and can press in and out slightly. I was thinking it might be missing a snap ring retainer but I couldn't find one at the site. I was definitely overworking the truck and it no doubt overheated. Is this just some safety mechanism that engaged of have I possibly really harmed the truck in some way. It appears to be working fine but I'd really hate to break dowm somewhere where the wreckers can't go.