How measure input impedance this differential amplifier in LTspice?
There are two principal ways to use a simulator to measure the input impedance of an amplifier - the direct way, and the indirect way.

The direct way is to measure the input voltage, and current, and take the ratio.

The indirect way is to put a resistor in series with the input, and note the drop in output of the amplifier, and do the sums from that.

With a differential amplifier, take care that you're driving it in the right way. If you ground one input and drive the other, you'll be seeing a mix of both the differential and the common mode input impedance. To see the pure differential input impedance, you have to drive the two inputs differentially.