I don't really know how to check it necessarily, but it has to be the freehub. It only happens when the wheel spins as if I'm coasting. When I pedal (cassette rotating), it looks totally normal. Stop pedaling, and I can watch the cassette undulate (looking at the biggest cog). It's sort of like watching an untrue wheel from the top. Only the wheel is true, so the only wobbling is at the cassette when it's still and the wheel is spinning.

I emailed a friend at the shop about it, thinking it might be an installation error or something. However, I can't detect any looseness in the cassette itself. It seems firmly and properly attached to the hub. So, I don't think the cassette is loose or busted (it performs well and it's new). It's just an unsettling thing to look at, and it seems to have gotten worse with this new cassette from the old 105 one.

I've at least seemed to fix the shifting problems, but I still don't like the looks of the wobble. I want new wheels anyway, but I want to keep these as my bombproof training set. I won't have $ for new wheels anytime soon either.