Have a high pithed whining in my 2009 Subaru forester when going slow. Goes away as I speed up. Replaced bearings but still doing it.

SOURCE: 98 subaru impreza 5spd transmission bearing noise

Sure sounds like a throw out bearing having too much room. Might need adjustment.

SOURCE: Procedure for replacing rear wheel bearings 2000 Forester

i'm assuming you don't have a hubshark,this is a tool that will alllow you to remove the bearing while the knuckle is attached to the car,you may be able to rent this tool at a auto prts store .if not you need to remove the entie knuckle,first remove the trailing arm bolt,then the lateral link bolt{long and stubborn bolt},disassemble the brake shoes and parking brake cable,abs sensor if equipped the knuckle to strut bolt{no marking necessary they are not eccentric} then you need a to use or borrow a press to remove hub and bearings good luck

SOURCE: 2002 Subaru Forester with grinding noise and

Sounds like the differential to me, when the power drops you are probably crunching a broken piece thru the ring and pinion gears. This can be very dangerous, could lock the back wheels up while you are driving down the road and get you or someone else killed.

SOURCE: My 1999 Forester Subaru wheel bearings are bad

Are you sure that they bearings are really bad, bad tires can also give you a growl noise from the bearings if they are different or worn badly. It should not mater where you take it to have them done, just that they use good quality parts and replace them properly. Hope this helps and have a good day.

SOURCE: Noise from rear of Subaru

Possibly brakes not re-installed properly or if bearings were real bad it could be a worn out axle....