Seems like you have two different versions of your ejb classes. Try to use the same jar file in both JBoss and the separate tomcat.
You are using a jboss-client.jar
or jbossall-client.jar from a different version
Thank you Adrian. That did, in fact, turn out to be the case. I recently upgraded my JBoss server but neglected to update some soft references in my IDE.
Thanks again for you help,