Are you on a hard surface, and more importantly have you turned while in 4x4? If so, try rolling strait for a while before disengaging and see if the clunk goes away. There's no diff between the front and rear and you can build up a bunch of tension in the driveline if you turn while in 4x4; especially if you're on pavement or rock, etc.. That can make a clunk/bang when you take it out of 4x4.