Thanks everybody - so almost everything points to the rear brakes. Use OEM rotors and pads. If the pads are SB/TNN (old OEM parts), you need metal cut away shim, if the pads are new OEM parts, no shim is necessary since that feature is already incorporated.

Just so I understand it clearly, the shim/new pad eliminates vibe due to a change in contact angle with the rotor surface, it sounds like? I can see how the pad can 'skip' along the rotor surface and create vibes, if sitting at a wrong angle.

It sounds like either my indy shop used an old OEM part, or a non-OEM part, even though I was told they use OEM parts. Neither makes me feel good. The rear brakes were done ~ 2 mo ago. Perhaps the pads just need adjustment. At least I'll sound like what I'm talking about when talking to them, which is as good as saving money.

One final(?) question is whether I need to get this done right away or not. Other than the possibility that this 'vibe' will induce more rattle inside the car by knocking things around and loosening them, how bad do you think it is to live with it for another couple of months? Others had to live with the problem for several months with no dire consequences, so I'm guessing it's ok.
I've been getting too many things done to the car(s) lately, including tinting, so need a break from spending money if possible.

Chances are, I'll make a call later today to a shop anyway, but...

Oh, another question - is this problem specific to FWD Volvos? I haven't had this in my 940.