The dom4j jar file packaged with JBoss do not contain the xpath components (I recall reading the JIRA associated with this change (was it in 4.0.3???) but could not find it again). You will have to package the dom4j jar file with your app (that is what I did).
I am porting my application from WAS to JBoss and getting similar problem, currently running with JBoss 4.0.3. I have lots of problem with classpath settungs infact I moved all the related jars directly under /default/lib directory but unable to resolve this problem.
I am failry new to JBoss so don't know its advance configuration. Appreciate any help/info on this matter