I have the exact same noise at the exact same spot in the road where a low spot is when I leave my house. The first couple times I would stop and see what fell off. Nothing. After awhile I stopped looking for body
parts left behind. What I have noticed is, I get the "Bump" after I have
lowered my tailgate and then raised it back, then hit the low spot. The only thing I could figure is, when I open and close my tailgate, it doesn't
align right but closes just fine. It takes the bump in the road to fully
align it with the 30 years old body. As a test, I open and closed the tailgate, then hit it with a rubber mallet, sure enough, "Bump". I guess old Bronco body's settle like a house does.
Hope this helps.