Sounds like the 'infamous' rear end clunk... It's generally the slip yoke in the trans and occurs when the rear end twists up slightly under load (Think of a mild case of axle wrap).
On the newer F150's with long wheelbase, it's a 2 piece drive shaft and it's occurring in the mid section of the drive line.
Don't have it with my 13 F150, but it did occur with my old 97 F150. It fixed itself after I got the aftermarket overload springs for it, as that helped the axle wrap issue it had as well as it's sagging when loading anything in the bed..
Good luck!
Mitch
