Have you tried a careful inspection with flashlight and mirror of everything under the truck? If two pieces are banging together like that you should see something flattening, making metal (bits of metal coming off), or pieces getting bent or smushed. Look above the axle and diff. How about the whole bed? How about jacking the truck by the bumper and dropping it quickly with a shop jack while you listen behind the truck. Or have someone drive it slowly over a curb or traffic bump while you stand by it.
