After 6 hours of headache, the ONLY solution is to download the WSDL files.
It will always throw an error 500, because this is not the wsdl.
Once you download the wsdls from Technical Resources -> FedEx WebServices For Shipping -> GetStarted (click on "Rate Services" and then "Download WSDL or XML"), in your script you will need to load your locally stored WSDL and everything will work like a charm.
$this->_soapClient = new SoapClient("RateService_v13.wsdl", array('exceptions'=>true, 'trace' => true));
Thanks Justin. This is what I eventually found out to be true myself, and as I read back over this original post, I never closed it out with the correct answer noted. Yours is indeed the correct solution, which I find after hours of beating my head against the wall as well. The FedEx documentation is horribly lacking in this regard.