I have a 2003 RX300 with AWD and 133,000 miles. After the car shifts into 2nd at 20mph I get a pronounced chattering in the rear btween 25mph and 35mph (it feels like I'm driving over railroad ties!). The faster I accelerate the more pronounced it gets where it actually shakes the car. After I get above 35mph it rides smoothly again. Any ideas??

 

Bad speed sensor on the ABS? I would definetly get under car and make sure the driveshaft support bearings are oK.....