There was a post I made regarding noises such as you described:
"I also went after the "wagon squeaks". I lubed the middle seat back latches, I lubed the rear sway bar mounts and the top of the shocks.
I also replaced the latch mounts (they were missing the rubber isolators) and adjusted them on the covers in the right and left rear (cover for the spare tire and the rear windshield washer reservoir /emergency kit). In addition a bought the round plastic pieces for each cover fingers at the bottom of the cover, that go into the floor.
Additionally I lubed the hinges and the lock plate. The rear is now quiet."
Found at: Wagon Sqeaks