The shift has to be the same for both Tx and Rx. As I recall, Airtronics is + shift, so the Rx must be too. It should be obvious that the crystals have to be the same frequency. What is even more obvious is that the crystal you plug in the Rx should fit and be compatible with it.
Tx crystal are another story. Generally you can't change them without re-tuning the unit. However, I believe you can change a Tx crystal in a Tx module as long as it's in the correct end of the frequency spectrum (hi or lo).
Ciao,
Eagle Al
