UPDATE: While the shop was trying to find the electrical gremlins in my fog lamp, I asked them to also track down the source of the vibration. Apparently it's being caused by the exhaust pipe rattling against the Kilby skid plate, so they checked all the hangers and found that everything was tucked up tight like it should be. They told me that there's like a 1mm clearance between the pipe and the plate at off/idle/park, but that the slight vibration associated with being in drive is causing it to contact. It's probably happening at speed too, but it's more noticeable when you're sitting still. They suggested cutting a hole in the plate to eliminate the rub, which I thought was a good idea given this problem and the concerns that Sue, I, and others have had with the potential for significant heat build-up with the Kilby plates. I'll take a pic when I get it back so you guys can see what was done.

Can't wait to pick it up to see how much better it drives/stops!