I had this problem, it sounds EXACTLY like what you are describing. It drove me crazy figuring it out, until I finally observed that the sheetmetal on the tub that one of my body mount bolts fastens to was partially broken out. Every time I hit a bump, the cracked sheetmetal would scrape against each other causing an ungodly screech. On my Jeep it was the body mount below the passenger side floorboard. I ended up taking the bolt and bushing out, reinforced the broken out mount with heavier steel, and reassembled. Fixed the problem.