Originally Posted by TIMZTOY
jack up all 4 tires off the ground, and have her drive 5-15 mph and you crawl under the car listening for the noise. youll find it !!!!! we do this at work all the time
Man, that scares me to death. I bet you guys have a lift that would prevent the truck from falling no matter what. Jack stands don't seem quite as reliable, especially when you're putting all that torque on them.
Another thought: pull 1 driveshaft, engage center diff, drive it. Still hear the noise? Put the first one back in, pull the second one. If the noise goes away, you have found the culprit.