My dad has a 2006 Subaru Outback that has had this issue ever since we took it to a mechanic to have the transmission fluid replaced around a year ago. We've taken it back to tons of places since, but they haven't been able to fix anything except things we didn't need fixed, so we decided to look into what the issue could be ourselves.
I originally thought it might be the torque converter, especially because our transmission fluid needs replacing much more than it should and I thought the converter could be disintegrating into the fluid, but the vibrations go through the whole car, not just the steering column, and it doesn't go away when you lightly tap the brakes. My dad reckons it could be the CV joint/axle.
Have you seen this before, and if so, any idea what it might be?