I had the same kind of shake. I removed the read brake pads and modified the shim by cutting out a square in one of the quarters. Since then I never get the shake. I still occasionally get the ABS light. The dealership also told me I needed a pump, but from what I have read in other posts, that is a common code when the card fails.

For me, the shaking was unrelated to the ABS light.

The instructions for modifying cutout are on bay thirteen

http://www.volvospeed.com/850rbrake1.htm

Go to the second page.