Just want to confirm for anybody wondering that the 2006 WRX is the same process. You do not need to remove the differential retainer to pull out the seal. For the WRX the seal looks like the old seal pictured in the original post, the one with the raised lip in the center. This caused me some confusion as to whether I had the right part or if I would need to remove the plate. I was having a pita time trying to get the passenger side in, so I put the old seal over the new one and pressed on it with a 36mm socket. The socket was not big enough to press the new seal without bending it, but the old seal over the new seal worked perfect.
Hope this helps anyone who was having trouble getting the seal to line up like I was.