Although that model has the upgraded drive shaft flex disc that even an idiot can't put on backwards, it still could be coming apart. Get the car in the air and carefully fondle each of the 12 bolts that hold the drive shaft in place. Don't just look at them: try to turn each one by hand. Inspect the two discs for signs of coming apart. Also check the center bearing for excessive movement, although your description doesn't sound like center bearing to me. If you don't find anything wrong with the driveshaft or any of the mounts, etc, there is one other possibility that you don't want to hear. I have seen a few cases where the problem was in the transmission. Also how old are the tires? When tires go for years without much use, they may look like new, but they are unravelling internally.

Good luck,
Peter

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