matching a differential port

Using low loss and temprature stable Baluns may be a solution.With Back-to-Back configured and connected baluns , the calibration is done and these are connected to the circuit.

For instance , consider a circuit that consists of two bac-to-back connected baluns which are connected very close on a double sided PCB .
In this case you may do a calibration. Then insert your design between Baluns' symmetrical pins and check it..

Another solution to buy a VNA that can capable to mesure symmetric network analysis but you may imagine that is very expensive..

That way as I've explained is valid up to 2.5GHz but beyond this limit , probably it should be used microstrip baluns but accuracy may not be very good..

Regards