I have been battling a similar problem for months. It occurs when the brakes are hot and usually during slow stops. It shakes the whole car and scares the heck out of the wife. To make a long story short, I finally took it to the dealer and took the mechanic for a ride. He immediately said he recognized it and recommended new rear pads and rotors. I had after market pads, and the rotor was nearly new. So I sanded the rotor and put new Volvo pads on the rear. Problem still there! I called him again and, even though the rotor was nearly new, he suggested new rotors, plus the volvo shims, which I had skipped since I had after market teflon shims. He indicated that their experience is that even though the rotor looks fine, and measures fine, it somehow changes via heating (turns a little blue?). So now I have the Volvo shims, volvo rotor, and I plan to do the job today.

If this doesn't fix it, I am giving the car away :) The good news is: I am nearly an expert at 850 brake jobs. The bad news is I am too old for this sh#*, but too cheap to pay the dealer $400 for a rear brake job.