I have been looking at the source code of run.jar (Main.class delivered in the JBoss4.0.3SP1 package) and in the code the "-j" option is handled as obsolete option:
...
case 106: // 'j'
{
System.err.println(programName + ": option '-j, --jaxp' no longer supported");
System.exit(1);
break;
}
...

However, in the documentation there is still reference to this option as follows which is little misleading:
"The JAXP JAR which is either crimson.jar or xerces.jar depending on the -j option to the Main entry point. The default is crimson.jar."

You can find that sentence in the following pages:
http://www.redhat.com/docs/manuals/jboss/jboss-eap-4.2/doc/Server_Configuration_Guide/Inside_the_JBoss_Class_Loading_Architecture-The_Complete_Class_Loading_Model.html
http://www.redhat.com/docs/en-US/JBoss_Enterprise_Application_Platform/4.3.0.cp07/html/Server_Configuration_Guide/Inside_the_JBoss_Class_Loading_Architecture-The_Complete_Class_Loading_Model.html

You may want to remove this sentence from the documentation. And/or, in these particular pages explaining how/which bootstrap libraries are loaded, you may want to mention "endorsed" lib folders to complete the class-loading picture of JBoss server at least in the context of JAXP.