Banging sound

I have a 1997 Ford Explorer, 125,000 miles. I hear a loud banging/clunking sound when pulling out fast. The transmission doesn't seem to slipping or anything. My husband just replaced the ball joints thinking that this was the sound, but, I still hear it. If I step on the gas fast-like I hear a loud bang seemingly to come from under the drivers side floor. I don't hear it anytime except when it is straining like going up a hill, etc. Do you have any ideas on what it could be. My husband tries to fix things himself if he knows what to look for.