For these funky vibrations, rumble, ghost walks, and other strange front end things, as a test you might want to try putting 4 adults in the car or 200 pounds of sand bags in the cargo area. If the funky stuff stops then the rear wheel alignment toe in needs to be decreased (with no weight in the car). The decreased toe in should help a little but this cars design will never be as stable as you would like it to be for highway driving because of the ride height. This is more so for vehicle designs like jeeps with aftermarket lifts ("aka" Impreza with a lift ). A car/truck could get too much rear toe in as air (highway speed, wind, turning, exc.) pushes up the car body and the Subarus rear lateral link arm will change the rear wheel toe in to compensate for the rise in body height. The adjustment will set the toe in as if you had passengers in the car. Makes sense. This information comes from an alignment shop with over 50 years experience.
After hearing "feels like a normal Crosstrek" from a few Subaru dealer techs for the same vibrations/rumble you all are experiencing, I had my MY14 Crosstrek (12K miles) aligned today from an general alignment shop ($129 out of my pocket) and after driving it for 90 min car feels a lot better but there is still a slight trace of vibration/rumble as you all posted. I will post again after some more driving time.