My rear end vibrates what i'd the reason ?

Of car or body?: If it's your car, you'd better ask a mechanic. If it's your body (your buttocks), you may need to be more specific. It could be just a sensation (paresthesia) resulting from something like too much physical pressure, but if it's something visual that someone else can actually observe, then it's more likely a muscle twitch (fasciculation). I doubt it's serious but if concerned, go see a doctor.