Calculating output impedance without measuring open-loop output voltage of a push-pull Class AB MOSFET Amplifier
Something seems to have gone wrong in your derivation. I used a symbolic math package (sympy) to solve the equations:
$$V_{out}\frac{R_1}{R_1+Z_{out}} = V_1$$
$$V_{out}\frac{R_2}{R_2+Z_{out}} = V_2$$
and this results in this expression for \$Z_{out}\$:
$$Z_{out} = \frac{R_1R_2(V_1-V_2)}{R_1V_2-R_2V_1}$$
Plugging in your values results in \$104\Omega\$.