if I add these two lines:

.save V(out1, out2)

.probe vd(out1, out2)

then run the simulation and at the ngspice type '`display`' i find the differential voltage plus out1 and out2:

Circuit: **.subckt untitled2-5

option SCALE: Scale is set to 1e-06 for instance and model parameters
Doing analysis at TEMP = 27.000000 and TNOM = 27.000000

 Reference value :  1.00000e+00
No. of Data Rows : 331
ngspice 1122 -> display
Here are the vectors currently active:

Title: **.subckt untitled2-5
Name: ac1 (AC Analysis)
Date: Fri Apr 29 10:32:15  2022

    frequency           : frequency, complex, 331 long, grid = xlog [default scale]
    out1                : voltage, complex, 331 long
    out2                : voltage, complex, 331 long
    vd_out1:out2        : voltage, complex, 331 long
ngspice 1123 -> 
ngspice-36+ done