What he said. I think you have one on each side. Far back corners of the truck.That goes on the very rear, on the body. It's the rearmost boxed section under the tailgate.
Lay under the truck on your back approaching from the rear. It will be toward your toes on the left or right side.
Just hit ATR at Windrock this weekend after popping it back in so hope it's still there 🤣IME the plastic shrinks with age then falls out when you hit a bump. Lucky you found it.
FWIW you could stick a short length of butyl rubber "rope" (Window Weld) on both the top and bottom before you snap it back into place. That way it won't pop out but will still be removable in the future if needed.
FWIW