It shouldn't be using
com.sun.org.apache.xalan.internal.xsltc.trax.TransformerFactoryImpl
but rather
org.apache.xalan.processor.TransformerFactoryImpl
(You need the real Xalan on your classpath)
See also instantiateTransformerFactory() at line 224 of https://github.com/plutext/docx4j/blob/master/src/main/java/org/docx4j/XmlUtils.java