The rear differentials in these all wheel drive Subaru’s pretty much don’t have many problems at all. I have seen them fail though. Usually failure is due to driving it too hard or running it out of fluid. It’s pretty common to hear pinion bearing noise but it’s never excessive enough to address the problem. When they leak this can cause problems though. These differentials only hold 1 to 1.5 quart(s) of fluid. It doesn’t take much for it to run dry and cause problems. Take a look at the failed ring and pinion gears below in the gallery. You can see the pitting and deteriorating gear face, this is caused from running it dry. So these differentials are easy to rebuild but not cost efficient if you have to replace the ring and pinion. Just to buy ring and pinion gears, bearings and seals is almost equivalent to purchasing a new one from Subaru. If your just replacing bearings then it may be more feasible. Another good option is to find a good used unit, like I said these differentials don’t fail that often so there are many good used ones out there. This is the route to go for this type of repair in my opinion. Just remember the differentials are matched to the transmission so don’t install a rear differential with the incorrect gear ratio. I first check to see if I need a limited slip or open differential, (some are and some are not) turn the pinion shaft and hold one side of the carrier. If they both try to move in the same direction then it’s a limited slip and if the other side turns in the opposite direction then it’s an open differential. Now remove the rear differential cover and count the teeth on the ring gear. If you have the same teeth count then it should work. You can just re-use the metal gasket for the differential cover, it won’t leak. Maybe clean surfaces and use some copper coat to be sure. Another way of determining the gear ratio is to turn the pinion shaft and count the turns on the carrier (input to output turn ration). This is where you get the 4.11/1 or 3.75/1 gear ratios. That would mean 4.11 or 3.75 turns to 1 on the carrier. Not as accurate this way that is why I recommend to just count the teeth on the ring gear to be sure. Just to be sure I will do both input/output turns and ring gear teeth count when matching gear ratios.