Each time I drive in 4wd i hear a loud clunk, mostly during slow tight turning but also sometimes just when starting to move straight ahead.almost sounds like a u joint but super loud. I have checked every thin out and all seams tight and not worn. I know whata cv joint sounds like when it's going and this definitely is not a cv joint. I has almost a ping noise to it like if you were to push a dent out of sheet metal. Any thoughts?

also it is mostly when driving in reverse and turning.