I'm afraid I don't know weblogic well enough to advise on how to configure it so that this JAR file is found.
You might experiment with loading Saxon directly: replace the call on TransformerFactory.newInstance() with a call on
new net.sf.saxon.TransformerFactoryImpl();
When you've got that working you can go back to using the JAXP factory loader assuming you actually need it (the direct load is much faster).
Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference