I also am having a problem with this. I need to use a newer version of the commons-httpclient.jar and when I include it in <jboss home>/server/default/lib and deploy my EJB, I get a java.lang.VerifyError. I asusme this is because the version in <jboss home>/lib is already loaded.
JBoss 3.2.1 and trying to use commons-httpsclient-2.0.jar
Try replacing the file in <jboss home>/lib
Put the jars you need in your EAR base and add it to the manifest of the jar that uses it. That will be used before the JBoss folder jars. Same idea with web-app where the WEB-INF/lib folder will override anything in the EAR or in the container or in the system classpath.
Thanks. I replaced the <jboss home>/lib with the latest version and it works fine now.