rear leaf spring u bolts loose

[texazranger07]

i noticed a bunch of clacking and banging around in the back of my truck for the last month or 2 and wrote it off to the ford flap on the back of the tail gate...til i checked it yesterday and realised the u bolts clamping my leaf springs together on the rear passenger side was loose...like way loose and that its actually wearing away at the center rod. sorry if i dont know the names of everything going on back there..needless to say these nuts dont loosen in the least bit, or tighten...so where do i go from here.... what could have caused this???