Leaking Trans Fluid

[Whitelm]

Has anyone else had this happen to them? I have a '71 k10 with a th400. When I drive it around just joyriding everything is fine. But when I have to use it for what it was intended, as a truck, and put it through the ringer, it leaks transmission fluid. Not just a little bit either. It does not leak when it is sitting, or when I take it for a casual drive. Only when I really have to make it work.
I have cleaned the outside several times trying to find out where it is coming from but I can't pin point it. The other day I could smell fluid so I stopped to take a look and found the front drive shaft soaked with fluid. Why does it only leak after hard use? Any ideas??