How would I measure distance with a transmitter/receiver pair?
If both devices can transmit and receive, then you could arrange a simplex system where when one unit receives a ping from the other it immediately answers, and so on.

Knowing the receive-to-transmit delays in each unit and subtracting them from the total time per transmit-receive cycle would allow you to get the time of flight of a pair of pulses and, knowing the velocity of propagation of RF, the distance between the two devices.

There's obviously a lower limit to the measurable distance,  but depending on what you need to do, the scheme might work for you and is sketched out, roughly, below: