Ok, I found it!

Using a few "DIY Rear Shock Mount" search results I learned which under-trunk plastic trim parts to remove to expose the rear shock mounts. Then had my wife drive me around again while I snoofed at the different parts.

The sound is unmistakably coming from the driver's rear shock. The shock mount looks brand new though. It looks to be in fabulous condition and all the bolts are rock solid. Never the less, all you have to do is touch the shock mount plate or the shock piston sticking out the end, and you can feel the Tok, Tok, Tok emanating right outta that part set.

The reason I was hearing it inside/behind the rear carpeted panel (and I could still hear it there too, clear as day) is because the metal unibody panel behind that (the car!) is basically acting as a drum head and echoing and even amplifying the RSM noise.

Last evening my driver's side window door regulator died, and so I had to take the car in anyway this morning. My mechanic and I were discussing the RSM noise too. He's thinking it might actually be a defective shock valve inside the shock and not the RSM. RSMs are very simple (hunk of rubber + two steel plates) and when they wear out you can move them around, see that the rubber is torched, and when driving they make a much louder "CLUNK" and only when you hit certain bumps. Shocks have two or three segments of the metal valve that are designed to handle different parts/segments of the shocks travel. THe 'early' part of the shock valve is meant to handle micro tiny variations in the road surface, and once you actually hit something bigger, the shock piston moves beyond that 1 or 2 little mm of travel and gets into the larger portion of the valve position that handles bumps. This might be why we hear the Tok Tok very rapidly, constantly, and the second you roll off driving the car, but if I hit something bigger, it's actually completely quiet.

These are replacement OEM Sachs shocks that went on 45k ago (4 years this month) and so they're way past warranty (bummer). Anyways, he's going to check out the shock, the RSM, they sway bar end links (tiny rubber bushings there), and the subframe bushings.

I'll post back on exactly what it was...