replace.js

var result = document.evaluate('.//text()',
                               document,
                               null,
                               XPathResult.ORDERED_NODE_SNAPSHOT_TYPE,
                               null),
    textNode,
    i = 0;

while (textNode = result.snapshotItem(i++)) {
  textNode.textContent = textNode.
                           textContent.
                           replace('replace me', 'with this');
}