ALPHABETS13:
When mine did that it was the joint on the rear of the front driveshaft., its not a u-joint, and I was told I had to replace the whole thing, so Im saving up for a beefed up one from Superlift.
It would make the reverse noise you mentioned, and when I turned into a parking place or something else at low speed, I would get this sort of chatter.... also when I would slow down at a red light and come to a stop it would chatter or grind....I thought it was the bearings, then the ball joints(which were worn out), and then the tie rods and R&P bushings and nothing helped until I pulled the front shaft and then it quit.
Also check the bushings on your differential, they are kind-of like motor mounts, and can clunk if worn out, but will only do it when you change directions in the tranny(fwd to rev and vise-versa) or when you let off the gas and hit it at cruising speed (thats what mine did when they wore out).
Mine is an AWD though, so a 4X4 will probably have the Constant Velocity Joint in the rear of the ft shaft, and that has Ujoints in it.
Hope this helps someone...