well for starters...you do have separate shocks and springs back there.
you could try and grease up the rear springs.
I have run into the past where the squeak came from the rear sway bar center bushings being totally gone. An that would cause metal on metal............squeak squeak... every time you go over a bump or down a bumpy road.
Or it could be your upper shock mounts. Try greasing those as well
heck...get yourself (2) 50 gallon drums of oil...build a pool around your car...fill it with the oil...let soak for 5 days...than drive it to see if you have squeaks than.LOL