rear knocking sound updated

hi everyone;
i have posted few days ago (rear axle knocking),this knock is driving me crazy,i have rechecked,retightened every single bolt from the rear suspension,while i was under the truck braking my arms off,my eyes rolled back to a single spot on the chassis rails.in the rear end of the chassis rail there is a center big bolt holding some kind of a rubber bushing and next to it from the left and right side two smaller bolts running the three of them as a line.i have tightened the center bolt and amazingly it turned almost a full round turn (he -man strenght).what i did is right?or this bolt shouldn't be tight that much?i can't measure the tork,i have just tightened with all force i can get.what is this bolt for?is holding the body to the chassis or something?could it make that knocking sound if it is loose one turn?checked and tightened the other side also.please if someone can give an explanation of this part and wht is the main use of it i will appreciate that.
i didn't have the time to drive my truck (i came black from under the truck and took a shower)
i hope it will solve this f---- knock coz there is nothing else i can do.all the rear bushings are perfect,everything is tight,exhaust is tight.what else can do this knock?
derek ward if ur reading this thread i have checked the uj"s and shafts everything is good no wear and tear.

i have checked also the 2 bushings of the trailing arm,there is a small play in the arm if u move it with a lever side to side,the bushing is not cracked,i am talking about the one near the center muffler where the bolt is far few centimeteres from the center muffler.

My rear knocking was the rear diff missing a couple of teeth on the rack and pinion