When you start JBoss you need to pass -Djgroups.marshalling.compatible=true to java.
FYI for any one reading this thread: http://labs.jboss.com/portal/jbosscache/compatibility/index.html is a good resource for tips/suggestions re: upgrading JGroups and JBoss Cache.
Thank you for your reply. I have the jgroups.marshalling.compatible system property already set to true though.I have confirmed this via the jmx console on both nodes. Is there anything else it could be?
Ah, I forgot. The -Djgroups.marshalling.compatible=true fix was added after 2.4.CR2. So you'd need to check out JGroups HEAD and build it, or wait for 2.4.GA, which should be out within a week.