Rear Knocking over potholes

[100%Saab]

For my 2009 9-3XWD I notice a thump, thump-thump, or thump-thump-thump when going over moderate potholes but not over ordinary road bumps or cracks. This occurs only on the left rear side. I have seen a blog post suggesting this could be worn top rubber suspension bushes which set on top of the rear shock absorbers. I am thinking about replacing the upper bushings for the rear shocks, part #12778198. The question is do I need the hexagon screws, part #11900029 and the nuts, part #11900441 also, or is all I need the bushings?
Any other suggestions? Thank you.

[100%Saab]

Did not need the screws. Used the old ones just fine. Replaced the upper brushings today. Problem appears to be fixed. 9-3 has tighter feel to it while driving also.

[100%Saab]

Replaced rear self leveling shocks at 58,000 miles when noise returned going over bumps. Driver's side shock was obviously leaking, but the other shock needed replacing also.