Why is my 1997 silverado Z71 leaking transmission fluid while pulling and not under normal driving?

The transmission in my truck is leaking fluid. It only does it when pulling something. I was pulling a 2 wheel dolly with small nissan pickup on it. Before I start pulling the truck, the transmission was fine no leaks at all. I drove it about 90 miles to get the truck without any problems. It only started leaking when i start pulling the truck. I had to go up and down alot of hills but they were not that tall. But the trans was changing out alot. The person that I bought the truck from told me that I need to put it in 4wd when pulling that will slow it down i thought. I have to put a quart of trans fluid in it when i got home. The next morning it drove fine and shift out right and without any leaks. Why is it leaking while pulling something and not driving regular? Please help...