I would like to know if you were able to find the solution for this. We are having a similar issue wherein hardware based load balancer is used with sticky sessions. So as far the request goes to same jboss node, the session is retained but as soon as request goes to another node, the session is lost.
you just need to set Clustering up (with adding <distribuatable /> to your web.xml), then it works fine (see http://community.jboss.org/wiki/HttpSessionReplication for further information). Mod_jk isn't needed as the session-state is replicated inside JBoss.
When your JBoss sends his internal IP, you can set proxyName/proxyPort in your HTTP Connector (server.xml). This way JBoss returns the Proxy-Address (your LB's IP) on getLocalName(), getLocalAddr() and getServerName().