I understand that 116K is a long time to ask favors from Subaru but my rear diff should not be having issue yet. I haven't had any unmatched tires. There is no leak on the rear diff. I got my 30/60/90 services. I don't know if the dealer (Eastside Subaru - Titus) did service on my rear diff.
Just to get a second opinion, I took my car into AWD Auto in Kirkland (indi-shop) and they had it for 2 hours and said that it wasn't the rear diff and that the noise was minor. I am however, getting my timing belt service done along with my water pump and cam seals, etc. Maybe the noise was coming from the belts.
I just got bit from the dealer that I have been going to for the last 4 years. I actually trust the indi-guy more so now because the dealer was trying to suggest getting a new car if the maintenance and repairs were going to be expensive.