java.lang.ClassNotFoundException is thrown because the loading
mechanism is looking for org.apache.xpath.jaxp.XPathFactoryImpl
and in Tiger it should be trying to find the package renamed class:
com.sun.org.apache.xpath.jaxp.XPathFactoryImpl.

To reproduce this problem use this program:
import org.w3c.dom.*;
import javax.xml.parsers.*;
import javax.xml.xpath.*;
import java.io.*;
import org.xml.sax.*;

public class Simple {
   public static void main(String[] args){
        Simple app = new Simple();
        app.run();
   }
   public void run(){
        Document doc = null;
        try {
            DocumentBuilderFactory df = DocumentBuilderFactory.newInstance();
            df.setNamespaceAware(true);
            DocumentBuilder bldr = df.newDocumentBuilder();
            doc = bldr.parse("foo.xml");
        }
        catch (ParserConfigurationException e){
            System.err.println("testmodeXPathAPI: " +
                " ParserConfigurationException.");
        }
        catch (SAXException e) {
            System.err.println("testmodeXPathAPI: SAXException.");
        }
        catch (IOException e) {
            System.err.println("testmodeXPathAPI: IOException.");
        }


        try {
            XPathFactory xpf = XPathFactory.newInstance();
            XPath xpath = xpf.newXPath();
            Element elmt = doc.getDocumentElement();
        }
        catch (XPathException err) {
            System.err.println("runXPathAPI_Q1: XPathException: " +
                err.getMessage());
            err.printStackTrace();
        }

   }
}

setenv JAVA_HOME <tiger_location>
set path = ($JAVA_HOME/bin $path)
javac Simple.java
java -cp . Simple

Output:
Factory Impl Name : org.apache.xpath.jaxp.XPathFactoryImpl
runXPathAPI_Q1: XPathException: java.lang.ClassNotFoundException: org/apache/xpath/jaxp/XPathFactoryImpl
javax.xml.xpath.XPathException: java.lang.ClassNotFoundException: org/apache/xpath/jaxp/XPathFactoryImpl
        at javax.xml.xpath.XPathFactory.newInstance(XPathFactory.java:61)
        at javax.xml.xpath.XPathFactory.newInstance(XPathFactory.java:35)
        at Simple.run(Simple.java:33)
        at Simple.main(Simple.java:10)
Caused by: java.lang.ClassNotFoundException: org/apache/xpath/jaxp/XPathFactoryImpl
        at java.lang.Class.forName0(Native Method)
        at java.lang.Class.forName(Class.java:141)
        at javax.xml.xpath.XPathFactory.newInstance(XPathFactory.java:58)
        ... 3 more