I have a 1994 4x4 with the 5.9l in it. Its a project truck and the transmission was slipping and had no reverse. But it didnt leak. I changed the filter, put on a new gasket, cleaned the magnet, and added 4.1 qts of atf +4. The truck seemed like it shifted a little better and i drove it for a bit and parked it and let it run for a bit and just manually shifted through the gears. There weren't any leaks or anything like that. I parked it in my friends garage and went back to college. I dont know how long it took but apparently when my friend went out to his garage 3 days later there was trans fluid all over. The truck hadnt been driven. Im going back this weekend and will be trying to fix it, I just want to know what could have caused it to leak out later when it wasn't driven.

 

Crack in the oil pan, bad pan gasket (likely because you dropped the pan, leaking dipstick o-ring, bad seal between tranny - t-case, crack in tranny, ect.