I have a 2000 explorer when i shift into reverse and step on the gas i hear i clunk type sound shift in to drive i hear the same thing. Sometimes when at a stop light and i hit the gas it does it every so often. Any Ideas what this could be?

 

A U-Joint with the needle bearing beat out of it. I would check the U-Joints in the drive line. If one is bad, replace them all at the same time.