I am doing a basic ajax request which fetches an entire page that then
loads into a DIV.  I would like to make some modifications to the
returned HTML.  Is it possible to convert the string into DOM object
which then can be manipulated, as I would using javascript?  Or what is
the best way to accomplish this?

The way to to this is get the responseXML from the Ajax object, instead
responseText.
Thus, you may parse the XML resulting and build the HTML elements from
it.

But, to get a responseXML valid, the server code may return a well
formed XML document, other way the responseXML on Ajax object will
aways return null.