it's been a long time since I experienced this error, so my memories are a little dim. As far as I can remember this error occured when I was running JBoss prior to 4.0.1 and struts 1.1. I found TagExtraInfo class in many jar files depending on the version of the JBoss. I'd say there is a colision of jar libraries and I'd focus on jar files named *servlet*.jar.
I suggest you do the following: Make a clean install of Jboss and deploy struts-blank.war file from the Struts zip file. Do not copy any jar files to the default/lib directory. If you'll be able to run the app, then try to deploy the application you're trying to migrate.
By the way which version of struts and JBoss are you using?