The rear of that car has what Volvo refers to as a Delta Link rear suspension. Imagine a flat rectangular box that has a line draw diagonally from one corner to the other, you end up with two triangles that when laid ontop of each other create a rectangle. The arms that you speak of in the rear suspension are just what I described, they each have two bolts on the base end of the triangle and a single bolt on the narrow or pointy end of the triangle. It is at the pointy end that they are secured into a block with a bushing and that block is in turn secured by the one bolt that I spoke of. In order to align the rear of your car you need to turn these bolts loose so you can get play in it. In my opinion it is a really, really piss poor design when it comes to adjusting it. The design is solid and robust and never really requires adjusting but when it does it is really a vague deal. I much prefer some kind of threaded set up or even ecentrics. It is really tough to guage how much you moved it or in which direction you are going. But again, the good news is that once they are set up they seem to hold their settings very well. In fact I aligned a '94 today and it was dead nuts on in the rear and only off slightly in the front. One thing though, when they first came out with this stuff in '93 it was mandatory that you replaced those darn bolts if you loosened them to do an alignment. A lot of guys do not adhere to that mentallity but I can tell you I have pull those bolts out in the past only to find them coke bottled all to hell and gone from some gorilla with a 1/2" impact.
Anyhow, let us know what they say at the dealership and let me know how much they are charging for a 4-wheel align these days.

Regards,
Mark