I had same problem while upgrading my applications from Jboss 4x to 5x .
first it was unable to find xerces throwing
then I change the parameter parent-first="true" in jboss-classloading.xml
this time it was unable to find strut's jar files throwing exception such as
java.lang.ClassCastException: org.apache.struts.action.ActionServlet cannot be cast to javax.servlet.Servlet
I had to do following work around
2) I moved all my struts jars , xerces etc use in my war\WEB-INF\lib to jboss-5.1.0.GA\server\default\lib
the application finally deployed successfully.