Output impedance of differential amplifier
I found this on the internet: -



It shows collector current versus CE voltage for several base currents. The graphs indicate the natural slope resistance of the collector. For instance, when \$I_B\$ = 40 uA, I estimate that a collector current range of 8 mA to 8.75 mA corresponds to a CE range of 1V to 8V.

This means the slope resistance is \$\dfrac{7\space V}{0.75\space mA}\$ = 9333 ohms.

This slope resistance, in parallel with the collector resistor (Rc) dictates what the output resistance of the amplifier is. Re will have a small effect on this of course.

If you are serious about finding out, the next step would be simulation.