Hi LA -
Thinking Sorrento. Been to Florence & Venice. Thinking a smaller city
since we will be In Rome for the tour. Two nights.
These two nights include the one on your (jet-lagged) arrival day? You're flying into Rome? And you'd need to be back in Rome in the early afternoon of day 3 to begin the tour? If so, that leaves you just one full day to explore a pre-tour location. I'm thinking Sorrento isn't a great choice as most ferries will not be running that time of year, and getting there could eat up most of arrival day, depending on what time your flight gets in. Late Dec. weather could also be wet and the coast isn't a great spot to explore in the rain.
Orvieto might be a better choice. The duomo and other churches will be open, and underground tours (http://www.orvietounderground.it/index.php/en/) run all year except for Christmas. With just one day and a short amount of daylight hours, it's not as if you're going to have a lot of time to fill.
https://www.orvietoviva.com/en/
Editing to add: you might also consider Naples...which is probably as far south as I'd go on arrival day. Depending on weather, you could opt to take the Circumvesuviana to Sorrento and back from there the next day, or go only as far as Herculaneum or Pompei and back, and then follow it up with the excellent archeological museum in Naples if it's a day the museum is open (closed Tuesdays).
https://www.museoarcheologiconapoli.it/en/