My 2007 Outback has developed this intermittent problem where it will vibrate at around 2000 RPM between 35 and 40 MPH with light throttle. Feels similar to driving on a rumble strip, only not as bad. Also seems to happen during a shift. Doesn't happen often. Mileage is around 70k and the car isn't driven hard. Most all of that mileage is highway miles at between 50 and 75 MPH.
The dealer drove it for a while and finally got it to do it once and said a transmission flush should take care of it. The previous flush was at around 50k if I remember correctly. I told them to go ahead and flush it while it was there for the cat reprogram and it hasn't done it since, but then again it didn't do it often to begin with.