I'm having a similar problem with my 97 850. I came up with an idea to narrow it down to ABS or mechanical problems. I removed the 30A fuse for the ABS pump/motor to prevent the ABS pump from running. The ABS light stays on now, which I expected. I drove it a few miles yesterday, and so far the vibration is gone. I'm guessing the vibration was the ABS system activating. I'll drive it for a few days with the fuse removed to verify, then I'll have to figure out how to troubleshoot to find out what's causing it to activate when it shouldn't.
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