Noise in rear dfferential



I have owned this truck for 9 years, and from the start there is a noise only when it shifts from 2nd to third in the back of the truck. I have checked the transmission,changed fluid and filter also transfer case changed that fluid twice over the years. Universal joints are good.My truck is very well maintained. So what do think is making this clunk? This noise only happens from 2nd to 3rd(atoumatic trans) Thanks