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hello welcome to the site
does this happen while accelerating or while on decel ?
at a slow or fast speed ?
do you hear any type of noise like a woomp woomp noise ?
the reason im asking is the tires might be separated and causing this
can you run your hand over the flat of the tire and see if their is any large uneven sections?
the brake rotors been warped will cause this only while braking.
if you can jack up the back of the car and spin the wheel and look carefully at the tire while it spins and see if you can see the tire wobble.
i have a strong feeling the tire is separated.
this feeling you have is pretty common when a tire separates
let me know what you find.