This seems to work. G=100MEG seems a bit much.
* Ideal Fully Differential OpAmp
* https://groups.io/g/LTspice/topic/50198770#msg46663
*
.subckt D_OpAmp inp inn outp outn vcm
R1 outp n01 1
R2 n01 outn 1
G1 n01 outp inp inn 100MEG
G2 outn n01 inp inn 100MEG
G3 0 n01 vcm n01 100MEG
.ends D_OpAmp
