Clunk sound when first take off.

I've got a 2000 Chevy Blazer 4.3L V.6 AT 2WD 2 Door. When I leave in the morning I hear/feel a clunk from front driver side when I first leave my driveway. It happens whether im in neutral coasting down my drive or in reverse. Also if I go slow enough out my driveway, it wont happen until I start driving down the street at about 4 mph. The clunk only happens once and wont happen again until I let is sit for a good 20 min. Doesnt happen at stop lights. And doesnt happen while turning. Happens every morning when I leave and every afternoon when I leave work. Occurs exactly when I reach about 3-4 mph whether coasting in neutral or in drive. Please help.