AC impedance or DC ?
DC case for input drive a V and measure the current its delivering, then ohms law.
Of course the diode, being non linear, changes its incremental R with its current. For out
short input sources to ground, apply a V to out and measure current, again ohms law.
You of course can use a DC sweep to get a look at various bias/V forced.....
For AC use a simulator, like Simetrix, that has a Z probe that will graph Z over frequency.
Here I do a DC sweep inoput V, and do 3 sweeps with a diode bias set at 1,5,10 mA.
So read off graph V and I out of sweep source, V1, and calc DCR. If I was smart I would figure
out how to plot it but its bedtime for old one.
Regards, Dana.