AVR sending message via UART
I had the same problem using an XBee-PRO XSC module with one of the Sparkfun boards and the answer I received from Digi technical support was:


  From the sounds of it, you have the radios in what we call Pitch mode. This occurs when Pin 6 is held low on startup or for long periods of time.  Try disconnecting Pin 6 and then power cycle. You should see the issue go away.


If you look at the XBee Explorer Schematic you'll see pin 6 is connected to an RSSI LED because that pin is used as an RSSI indicator on some other modules. For the new revision of the board you'll see there's a jumper SJ1 that you can cut to disconnect that line which should solve the problem. I had an earlier board without that jumper so instead removed R7 for the same effect.