no communication when jumper settings switch to xbee from usb

Hi I am a student working on an arduino robotics project. My project requires me to use a computer connected a Xbee beacon ( board) to communicate wirelessly with XBee shield mounted on a Arduino Diecimila which will be mounted on the robot.

I am able to get the beacon to communicate wirelessly with the XBee shield if they are on separate PCs and only when the jumper setting for the Arduino board is set to USB.

However when I switch to XBee jumper settings, the range test on X-CTU shows that there is no communication.

info: Modem XB24; Function Set XBEE 802.15.4; Version 10CD

Any assistace is greatly appreciated. thanks.

1 week into it and
I am in the same boat, have two unos(had to disconnect the main Chips on each to make the uno talk with XBEE2 pros TX/RX via USB to serial monitor on XCTU GUI. I am scared to switch the XBEE SHIELD(s) over to XBEE setting. everytime I change setting I have to unplug the shield and XBEEs then reconfigure the port settingsā€¦ there must be a more simple way. any input would be great.