Clipping observed across small resistor when attempting to measure op amp output impedance
$$\frac{2 \rm{V}}{27 \Omega}=74 \rm{mA}$$

Based on the datasheet, the LF411 is not able to supply more than 25 mA (at 25 C temperature).

Here's the relevant figure from the datasheet:



You can see they don't even specify the capability below 100 ohms, but it's rolling off fast enough to tell you "don't go there".