Are the wire whip antennae being bent in any way?
These wires are 1/4 wave antennae that use the XBee PCB as a ground plane to virtually create a 1/2 wave antenna. If the antenna is bent, then the effectiveness of the antenna is severely diminished.
Also, speaking of ground planes, what is directly below the XBee? If there are metal objects, then this could cause issues with the 1/4 wave "mirroring" that occurs with the XBee PCB plane.
On the ground vehicle module, is the antenna in open space, or is it enclosed? If so, what is the material of the enclosure?
Are both modules oriented in the same fashion? If one antenna (full module in this case) is 90 degrees out of phase from the other, you will automatically loss 12dB of signal. This is very significant.