Worn rear suspension or shock bushings. Have the bushings inspected and replace any signs of damaged or worn suspension parts.
Good luck and keep us posted. Almost forgot, don not oil and bushings. Another common squeak make is the rubber stops on the hood and tailgate. open them up and look, there are rubber bumpers that they both hit. When the truck flexes a little on bumps they can squeak. Put a little lube on them and see if the it helps. Also remove your retractable cargo shade from the rear of the truck and see if the squeak goes away. The cargo shade end caps are known to squeak to the metal cassette of the cargo shade sometimes. If removing the cargo shade doesn't make the noise go away, let me know.
Post back and let us know if any of these things helped.