Take a look at your rear shock tower mounts. One, if not both are probably torn. This sounds like the beginning stages of the issue. Eventually both will tear, and you'll hear the sound all the time.
Go to the truck and lift up the carpet around the wheel wells. If there is an issue with your mounts, it will be obvious.
The other problem this could be is the rear control arms. Have a look at this too, but I'm gonna say it's your mounts almost for sure.