Yes, you connect the ground of the network analyzer test cables to the ground on your test board. The center conductor of port 1 cable goes to one side of the differential circuit. The center conductor of port 2 cable goes to the other side of the differential circuit. I use this method all the time to match differential output of a mixer into a differential SAW IF filter. I take the "single ended" S-parameter measurements, convert them to differential in @//D//S, then do the matching in the simulator, then solder the matching components down on the board. I can't think of any easier way to do this.