tmurray50,
I have raised a request for the dev team to include support for Struts 2.3.1.2.
As a workaround you can create a user library to include the struts 2.3.1.2 jar files. Go to Window > Preferences > Java > Build Path > User Libraries to create a new user library. Add the Struts 2.3.1.2 jars to the user library.
When adding the Struts capabilities to the project, you can select the User Libraries to add to the project’s build path instead of MyEclipse Libraries. For existing Struts 2.1 project, delete the MyEclipse Struts library from the build path and add the new user library.
Let us know how it works for you.