I had the same issue on my 2008 9-5. The rear mount bracket does not have the hole that a socket can drop through to access the 3rd (blind) 10mm bolt. I also had to remove the bracket to get at this. To remove the bracket, you take the 16mm nut and thick washer from the top of the mount from the top side. You can then remove the other three bracket bolts from underneath by reaching on the top side of the cat with ~9"-12" extensions - the bracket bolts are 18mm sockets. The bottom most socket is close clearance to the cat, so you can just use a regular wrench. Once the bracket is off and out of the way, you can remove that last 10mm bolt. It adds about another 30-45 minutes to the job but not too bad.
Just FYI - as I was doing this, I noticed that one (maybe two) of the bracket bolts was actually loose! It may have been the source of the clunking and vibration as well. The third bolt was still locked in, the 2nd was too easy to break in my opinion. I still replaced the mount, but it did not show any signs of damage that I could tell. New mount (or tight bolts) took care of the problem. I had to have my steering gear replaced last year and I know they have to take that mount out to do it - mechanic might have left it loose... Anyway, all done.