I have a 240Z build date 12/70 and I have what I think is the series 1 z-car rear end vibration. To date I have replaced all bushings, struts, and u-joints (including half shaft u-joints). I have heard that later 240Z's had there rear differentials moved back 2 inches and the driveshaft lengthened or shortened (can't remember) to curb this problem. Is there a kit or mod that can be done to stop this horrible vibration. Between 65 and 70mph I get a solid rumble in the rear. Thanks, Colin