I am using saxon8.jar within jboss. With jboss3.2.x I solved this by adding an entry to application.xml of the ear file.
But this doesn't work with jboss 4.0.3 what I am using now. I learned (in this forum) that this is not the correct way to include a jar file. So I tried adding it to the manifest file of the jar which is using saxon:
Class-Path: . saxon8.jar