AJAX post returning 500 Internal Server Error on Live Server

I created a script where a user can select from a variety of

shipping options for an order from within a jQuery dialog box. The

shipping rates are delivered via UPS & FedEx. Oddly enough, the

script works fine on my development machine, but on the live server,

Firebug returns a "500 Internal Server Error"

Here is what my jQuery looks like:

jQuery("#select-shipping").click(function(){
// Open the dialog, start with the "loading page"


$("#shipping-select").load('func/shipping_load.php').dialog({
bgiframe: true,
autoOpen: true,
position: 'center',
title: 'Select Shipping Method',
width: 400,
height: 400,
modal: true,
buttons: {
'Apply Shipping': function() {
var bValid = true;

if (bValid) {
var fship =

$('input[name=shipping]:checked').val();
var arr =

fship.split('-');
var shipPrice =

arr[1];
var shipMeth =

arr[0];

var subOrder =

$("#d_total").val();
if

($('#customer-discount').val() != 'false') {
var minDis =

subOrder * ("."+$("#customer-discount").val());
var nTotal =

subOrder - minDis;

var orderTotal

= nTotal + parseFloat(shipPrice) + parseFloat($("#tax").val()); /*

Order Total minus customer discount */



$("#order_discount_amount").val("-"+minDis.toFixed(2));
}
else {
var orderTotal

= parseFloat(subOrder) + parseFloat(shipPrice) +

parseFloat($("#tax").val()); /* Order Total, No Discount */
}


$("#order_total").val(orderTotal.toFixed(2)),


$("#shipcost").val(parseFloat(shipPrice)),


$("#shipping_method").html(shipMeth);



$(this).dialog("destroy");
}
},
'Close Window': function() {
$(this).dialog("destroy");

}
},
close: function() {
$(this).dialog("destroy");
}
});

// This portion actually runs the PHP code to get the

rate results from UPS & FedEx
$.ajax({
type: "POST",
url: "func/shipping_selection.php",
data: "shipid=" + $("#customer-ship-id").val()

+ "&cartid=" + $("#cart_id").val(),
success: function(html){
$('#sLoader').fadeOut("fast");
$("#sLookup").html(html);
}
});

});


The dialog opens a loader page (with gif) and that portion works fine.

Once the AJAX post is complete (shipping_selection.php), it is

supposed to update the "sLookup" div with the HTML from the

shipping_selection.php page. Like I said, it works perfectly on my

testing server, but fails on the live server. Both are running PHP

5.2xxxx.

Also, notice in the attached images that the 'shipping_selection.php'

is actually working...rate results are returned from both UPS and

FedEx, however something is failing on the AJAX success function.