Short of buying or building your own differential probe, you can use the MATH feature of that scope to create a CH1-CH2 trace. In this case, you would use two probes, one on each of the two nodes that you want to make a differential measurement of (with both probes' ground connections going to ground/earth of your circuit). Then, use the MATH to create a math trace that is CH1-CH2.
The old analog scopes used to be able to this in hardware by selecting "ADD" as the vertical mode, and selecting "INVERT" on CH2...