2004 Ford F-150 Lariat New Body

Lately I have been going between 55-60mph and
my truck best would start jumping up and down, I
thought it was the rearend and replaced it, also
replaced drive shaft, tires, stock rims, truck bed,
front and back shocks, brakes, rotors, drum brakes,
and the only thing I have not changed was the leaf
springs, this bounce is driving myself and my
family nuts, has anyone else had this issue?  And if
so, what should I be doing to fix the issue? The
jump goes away once I put weight in the truck bed
itself or tow a trailer. Ask me any questions so I
can get a better idea. Thank you for you help.