After rotating your tires around to see if that changes the vibration, I would crawl under the car and give your driveshaft a look. Check to make sure that the six bolt that attach the driveshaft to your differential have not broken. And also see if you have any slop in any of the bearings (center support bearing, as well as the differential drive flange bearing) My bearing in my differential finally blew. So this weekend ill let u know whether or not my vibration has gone away.

I had the same symptoms as you are speaking of for about 10k miles. I would not get vibration in accelerating or braking, but only after the car had warmed up and had been going down the road for about 5 miles at 60-65mph. If you lean your leg against the tunnel do you feel the vibration more than in the seat?