First time Subie owner. I bought a 2010 Legacy Limited used. At first it was love through and through, however lately I am disgusted. After time, approximately two years later, an intermediate body shake will show up on the highway around 70 MPH. More of a body shake than a steering wheel shake. It fluctuates to near nonexistent, to my butt doing the "hippy hippy shake". It really has me stumped as we just installed new tires (Road force balanced as well, per recommended by Subaru). Alignment was completed three months ago and the track is true. Any suggestions?