2000 Ranger 3.0 4x4 4r44e 7.5rear end. My friend and I have the exact same Ranger. He has had a clinking type noise from the rear since he bought the truck. Not a clunk. He took the rear cover off to check it out. Everything looked good. I got there and slid under it, he turned the shaft and there is something inside the shaft 6 inches from the rear u joint. As you turn it something is riding up and falling back down. It always stays about 6 inches from the back and does not slide back and forth. I have never seen or heard anything like this before. No drive line vibrations, and it only is heard at speed under 5mph. Anyone have any idea?

 

No, not something I have read about, but most likely something left inside when it was made
Yes, noise would go away once the driveshaft was rotating fast enough to hold the debris in place by centrifugal force

Only fix would be to swap out drive shaft, well cheapest fix, lol, he could have it cut open and re-balanced after debris was removed