Before you go and spend your money on an xbee that you may not be able to use, see what pins are in use by the other shields, and what pins the xbee needs to operate. If anything, get a Bluetooth master and slave module, you can get them on ebay for about 15-20 dollars. OR maybe a RF transceiver again ebay. I have both the BT modules and a RF transceiver, the RF transciever I got for 4 dollars on ebay (B.I.N.).
If you want, you can atleast test your code with a BT or RF and then move onto an xbee or something with more range.
Is this what you have?