[jQuery] How to post modified XML document whose source was an AJAX get

I'm trying to take some XML I receive from an AJAX call (via a "GET"),
manipulate it, then POST the document to an ASP.NET page. My "get"
looks like this:

var createNewUsersExtendedAttributes = function()
{
$.ajax({
type: "GET",
url: "<someURL>",
dataType: "xml",
cache: false,
success: processExtendedAttributes(xml),
error: function() {
// handle error
}
});

}

processExtendedAttributes = function(xml)
{
// set a member variable in this class
userProfileExtendedAttributes = $(xml);
}

Elsewhere in my code, I manipulate nodes in the member variable
"userProfileExtendedAttributes". All of this works great. However, I
have problems when I want to take that XML document and do a POST with
it:

$.ajax({
type: "POST",
url: "<someURL>",
processData: false,
contentType: "text/xml",
// remember, the userProfileExtendedAttributes
variable was assigned in the above success callback using $(xml)
data: userProfileExtendedAttributes,
success: function(data)
{
alert('success!');
},
error: function(request,status,errorThrown)
{
alert('failure!');
}
});

I've played with the usual suspects (processData, contentType, etc.),
and they are not fixing the issue. If I debug the page to which I'm
posting (or look at HTTP headers being sent to the page), I get
nothing in terms of posted data (note: if I put simple name/value
pairs in the data parameter, I do see those). I understand that when
I call the function that uses the post code above, the member variable
"userProfileExtendedAttributes" is NOT a string representation of the
XML document (it's actually a reference to a jQuery instance of an XML
document). I believe this issue is happening because I'm not sending
an XML string to the page I'm posting to, however, I cannot for the
life of me figure out how to get this XML document serialized so that
I can post it. Any help would be greatly appreciated!