u joints will have a shake under accleration.....at any speed not just 3000 and up....so when you unload the drive train by pressing the clutch in the shake is worse?????Like you are coasting right.....try shutting the car off and coast it...see if it shakes then...may have to rebalance tires.....is it in the seat or steering wheel....seat will be rear tires...steering wheel will be front tires....