got it. Thanks everyone. Here's the steps I took in order.
1.) copy the \Steam\steamapps\common\Galactic Civilizations III folder to a backup location
2.) go into Steam and remove the local game data from there.
3.) begin reinstalling GalCiv3, but create a new Steam folder on the SSD (Steam will only install into an empty folder)
4.) when the download starts, pause, then exit Steam
5.) copy the backup Galactic Civilizations III folder into the new \Steam\steamapps\common\ SSD directory, overwriting any existing files
6.) start Steam
7.) do NOT launch it from Steam -- instead navigate to the new \Steam\steamapps\common\Galactic Civilizations III folder and run 'GalCiv3.exe'. It will start in Steam, then verify the new local data.
8.) I did have to download a little bit, anout 110MB, but that's exponentially better than 3.8GB.
Thanks, all, and happy gaming!