Rattling noise from rear upon slowing down!

OK guys i have a rattling noise coming from the back of my truck when I slow down, like slow enough to make a turn is the only time you can hear it. I've been all under it, had some buddy's drive it while I've rode in the bed to try and pin point it but so far nothing. It almost sounds like something is loose, but i cant feel or see anything that is. Has anybody else had a similar problem and happen to have figured out what it is? It's a 2008 double-cab short bed, no lift or anything, just getting aggravating. Helppppppp!!!!!! And another quick question, passenger seat has a pretty violent shake upwards of about 70 miles an hour, probably an out of balance tire right?

 

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