It's official - dual chains.

We replaced both chains, and all accessible guides, plus the main gear. The teeth were worn to pointed tips instead of flat. I did not read any past posts about this, but suspect for $30 it is a good idea.

Also replaced the O2 sensor, some oil filler tubes, and a few misc. items.

End result - smooth and tight, with no more vibration. I am hoping to get some better gas mileage with the new O2 sensor .... time will tell.