Feels like the back end is fishtailing/floating when the road isn't real smooth

inspect your u-joints and driveline components, lift the car and try "wiggling" the driveshaft. if you hear a metallic clinging sound, look closely at the snap rings and u-joint mating surface. these joints from factory are quite weak and are not servicable (greasable) have it replaced for a servicable unit. be mindful of the bolts that attach the driveshaft to the differential, they can come loose apon occasion but this is quite rare, also do you have aftermarket rims? or locknuts? a couple grams weight difference on a single rear lug can cause a 2200lb car to shake at these speeds. also check integrity of rear shocks, if you can press down on the deck lid and depress the car with ease and it "bounces" back your shocks should be replaced