Okay, I figured out why Save For Web was greyed out. Apparently I had set up a Large Canvas document and upon further research, I found some documentation indicating that some export options are not possible from Large Canvas files.
It would be nice if there was the ability to change this option AFTER a file has been created. Since it's not an option, I had to create new documents with the Standard Cavas size, then copy and paste artboards from this file to the new ones that do allow Save For Web. Hope this helps others who stumble upon this issue.