direct traffic based on information from the XML request

Okay, well there's an XML xpath way to do this with the XML_CONTENT_BASED_ROUTING event and an XML profile, but for such a small set of data that's probably more complicated than a single string parse.

when HTTP_REQUEST {
    if { [HTTP::header Content-Type] contains "xml" } {
        HTTP::collect [HTTP::header Content-Length]
    }
}
when HTTP_REQUEST_DATA {
    set number [string trim [findstr [HTTP::payload] "" 8 ""]]

     enable for testing
     log local0. "number = $number"

     now that you have the number you can add the switch code to find the last digit

}

I'm making an assumption of course that the XML request has a Content-Type header with the word "xml" in it, and I'm also not accounting for what might happen if the request isn't xml, but I hope this gets you going in the right direction.