I have a Chevy Venture Minivan. Its a 2000. When i do 65 or more my seats in the back shake and rattle very bad. I took the car to the shop and had about 1500.00 dollars worth of work done. Shocks,wheel bearings, ball joints,new tires, alignment, shims put in the back. new brakes and rotors and tie rod ends put on . the shaking wont stop... any ideas what it could be caused from?