Sorry, I mis-stated that in my post. I was looking for "UML 2.1 Diagrams' and it was not there despite what looked like a successful install of the plugin.
However, I did ultimately resolve this by re-installing STS into a new location. (I promise I tried this BEFORE posting here, but I must have polluted the install somehow and it didn't work.)
Sorry for taking up your time with this, and thanks for your reply. The following ultimately worked for me:
1) Install a fresh STS into a NEW location using installer (w/o groovy in my case)
2) Open STS (make sure to use the shortcut pointing to the version you just installed!)
3) Before doing anything else, select "galileo" from the update site, select "Modeling->UML2 Toolkit" and install
4) Restart STS
5) To verify that UML2 Toolkit is installed: Select "File->New->Other" and look for the menu item "UML 2.1 Diagrams"