would I likely to find that info as part of a standard data sheet for a diode ?

You will likely find a graph indicating the voltage drop across the diode at various currents. The impedance at a particular current is given by the slope of the line tangential to the curve at that point.

That is a fancy way of saying exactly what Resqueline said, and would be familiar to you if you recall the limits stuff you may have done prior to doing differential calculus.

An

example for a diode at random. There is a graph showing typical forward voltage vs forward current.

You will note that at low currents, a relatively small increase in current produces a comparatively large increase in forward voltage. As the current increases, the forward voltage is far less sensitive to changes in current. This shows that thte diode exhibits a decreasing impedance with increasing current.

Note also that the graph is log/linear so you can't simply measure the slope. You need to read values off the graph and calculate as per Resqueline's instructions.

It's also worth pointing out that these curves are for a typical device at some constant temperature. In the real world the actual values will change between devices and with temperature.