I'd find it very unusual if only your muffler (rear muffler) needed replacement. Usually everything from the "cat back" needs changing, which you won't find out until you actually do the job.

Volvo sells a kit with everything to the catalytic converter; tailpipe, rear muffler, axle pipe and front muffler with all clamps and hangers in a box for roughly $200. It goes together smoothly with a minimum of tools.

FCP Groton and others sell a kit made by Starla (Walker) for half that price which is a lot better than half as good as the Volvo brand kit. I've used both several times.

Doing the whole thing is worth it. It is easier -- you only have to remove one rusty connection: I typically swing a 2 pound hammer into the front of the front muffler to disconnect it from the cat pipe, and pull the whole mess off in one rusty piece. Then assemble the kit and drag it under the car, get the front muffler started on the old cat pipe and then stuff the pins on the rear muffler in the hangers to hoist it in place. The trick is getting the donut hangers (round cross section rubber rings) around the ears of the front muffler and body mounted bracket. My solution is to use a bottle jack to lift the front muffler to the floor.

Sometimes I've needed a cold chisel to peel the old muffler off of the cat pipe if it was really rusted. All bets are off if you have some aftermarket Midasized or George Foreman exhaust system. However you do it, the second time it will only take 1/2 hour.
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Art Benstein near Baltimore