So just to clarify things, you are saying that if I change the baud rate on the Uno, I will not get a lot of junk data? What I mean is, I was under the impression that whenever two devices (say an Uno and a Mega) try to serially communicate with each other with mismatched baud rates, I would get a lot of nonsense displayed on the serial monitor.
The Mega and the car thing talk to each other at some baud rate.
The Mega and the XBee talk to each other at some baud rate.
The XBee on the Mega and the XBee on the UNO talk to each other at some baud rate.
The XBee on the Uno and the Uno talk to each other at some baud rate.
The Uno and the PC talk to each other at some baud rate.
None of the 5 baud rates needs to be the same. But, the baud rate for each end must be the same.
It still seems like you are using one serial port on the Uno to talk to two devices - the XBee and the PC. That won't work. If the Uno is doing nothing more than attempting to relay information from the XBee to the PC, then it needs to be running this sketch:
void setup() {}
void loop() {}
If the Uno is to be doing more than acting as a relay, the XBee can not be connected to the RX and TX pins. They are for communication with the PC.
Since the Uno only has one hardware serial port, you need to use SoftwareSerial to communicate with the XBee.