As others have mentioned, there is not enough information here, to give a complete answer.
Things like, how are the Servos connected and running? My assumption is you are using the Servo library.
Also as mentioned, not all servos are exact at 1500us as the stop position, so may need to adjust. Also some joysticks are not good at having a consistent center-point. Again not sure how you are controlling. I am assuming using analogRead? Assuming you don't scale it, that implies that center will be near 512. On some of these you may find that the actual center one time is 520 and on some when you move it and release it, it may come back to 515 or maybe 508... So we often have what we call a deadzone or deadband where we say that anything in the range of 500-524 is considered to be logical 0... The worse the joystick the bigger this range needs to be...
As for XBees, how are they connected? Shield? Sparkfun Regulated explorer? Using Hardware Serial Port or Software Serial? The reason I ask, is they may all react with each other. For example if you are using the Servo library, it uses specific interrupts and timings to generate the pulses for the servos. However these will get off of something else is handling or disabling interrupts. With Hardware serial, the interrupt processing is minimal, but with Softwareserial, interrupts are disabled during the complete output or input of a character, which can really screw up a servo signal.
Then there is the obvious things to check like how are sending the data between the two XBees. In Serial Replacement mode or in Packet Mode. I will assume that in simple text mode. Do you have code to handle knowing when a packet of information arrives? Do you process it quick enough to not lose data... There are some real simple XBee based remote controls out there like the Arbotix Commander(http://www.trossenrobotics.com/p/arboti ... ad-v2.aspx
), which uses a real simple protocol. There is more complex systems, like the DIY XBee remote controls which there are lots of posting talking about that on this forum (but no real kits for building them). My current code uses the XBees in packet mode and I have my own protocol set up for them....
Hope that helps.