I believe that your Subaru has a transmission issue. There are quite a few things that will make a grinding noise. Wheel bearings, worn out brakes, damaged brake rotors or drums, loose or damaged brake calipers, defective hardware kit (for the brake shoes), torque converter, flywheel, U-joints, and CV joints. The interesting part of your symptoms is that you can only hear the grinding between 30-60MPH. I think you need to continue testing and inspecting components, one by one. You have all ready narrowed your search down. You know that the Brakes, CV joints, and Bearings are fine. So now you need to focus on the drive train. Inspect as much as you can from the flywheel, to the transmission, the drive shaft and rear axel, and the output shaft and front drive axel. If you can not find it yourself it may be time to send it in for repair. Let me know if you have any questions or new information, good luck.

That is odd,I would check the emergency brakes ,Can't see 'em with just the wheel off,(small drum brake inside the rear rotors) If indeed the wheel bearings are ok ,thats my only idea.

When the wheel bearings were checked, did the person who checked them loosen the axle nuts on each one before checking for play? I ask because the axle shafts and axle nuts are basically clamping down on the wheel bearing, and you won't feel any looseness whatsoever if the axle nuts are tight. If you can get the car on a vehicle lift so you can drive the car in the air with the tires free to spin up to the trouble speed, you can get an automotive stethoscope relatively cheap and listen to the wheel bearings, rear differential, drive line center support bearing(if equipped), and transmission. It will be worth it to have an independent shop just diagnose the noise for you, then you can fix it yourself.