I hate to resurrect an old thread but.......I just had this same problem, and have finally gotten it fixed. The specific symptoms I had were intermittent vibrations, at tire rotational frequency, only under acceleration, worst at 55, returns again at 75. It took a full year for this to progress to the point where it would nearly change lanes at its worst. After much troubleshooting, we had to replace a motor mount, and both axles, now it is smooth as butter.........I hope this helps!!

Michael