Question about VCXO w/ varactor diode.

Is it possible to switch a resonant circuit comprised of a crystal and a varactor diode back and forth beteen an RF input and a L.O. circuit for transmit and receive?

Yes. But there is a fair amount of turn-on time before the oscillations start up and stabilize.

You have to make sure the active part of the oscillator is set up to work at both frequencies.

Would.t it be the same frequency for transmit and receive?

If you need the same frequency for both, then use one crystal and do not switch it!