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If the rear camber is out of specs and/or the toe can't be adjusted into correct specs, your problem is most likely due to bent rear trailing arms. If one looks up at them when on a hoist (for example), you can usually see when they're bent as new ones are perfectly straight.
I've seen them bent a few times on 850s and although I don't know why they get that way (accident, bad bumps, etc ?), the only cure that I'm aware of is to replace them. They're not cheap and it's about a 8 hour job (book flat rate time) including alignment, to my recollection it'll cost you around $800 ?(guessing here).

Hey there!
I have a 96 r wagon with the identical problem.
When I took it in to have the alignment done the mechanic adjusted the toe in and that corrected the problem.
hope this helps
klaus

Whenever getting an allignment, ask for a before and after printout, as well as manufacturer's recommendations. If they won't, go somewhere else.

Do you have any feathering? The rear often has too much toe in, which could be causing your problem, which you could easily tell by checking for feathering.

Why did the mechanic tell you they can't get the car alligned? What was the reason given? Sounds more like they just don't know how to allign the car. Trust me, that rear delta suspension is not something you want non-volvo trained people to allign.