getting infinite output impedance in Ltspice but not by manual analysis
.TF just doesn't seem to work in the expected way for the output impedance calculation when current is measured. 

If you take that same circuit and change the .TF line to .TF V(vout) vin, it calculates the output impedance correctly. 

The book CMOS Circuit Design, Layout, and Simulation, from whence your example originated, it appears, has a cryptic 'explanation' that makes little sense to me:

 

For example, if you put resistors across the voltage sources, the output impedance is still open. 

This problem has come up before on this stackexchange and on Dave's eevblog, where it received some attention but no definitive result. Whatever it is, it is baked into Berkeley SPICE and not an artifact of LTspice.