get a carfax. any accident hard enough to do that is likely recorded.

no amount of trailer bushing wear right down to the rubber in the bushing being gone will do what you have occurring. shot trailer bushings will reverberate through the cabin and make the sound unpleasant to unbearable. as metal on metal will no longer be isolated.

bad shocks down to shot shock will also never come close to do what you have occurring.

there is no rear wheel alignment possible and any mechanic who claimed to do this for you screwed you out of some money or is an idiot, you decide.

if it was my car i would place a floor jack under the rear axle and lift it up until both rear wheels were OFF the ground.
chock the front wheels in front as a precaution have someone start the car and place it in drive with no gas just idle speed. now watch the rear wheels as they spin, look for out of round or any odd ball behaviour.

get a long tape, lock the front wheels dead straight ahead. measure diagonally from the same bolt on the ball joint each side where it bolts to the strut tower to the same bolt on the rear end of the trailer arm each side. you can pick other points as long as they are consistent and the front tires are dead straight ahead. just to confirm your measurements lower the axle tot he ground and repeat. if the distance per side varies in a meaningful way you have a problem.