What does it mean for an input impedance to be "differential"?
It depends on how the inputs are designed. The instrumental amplifiers have very high input impedance (almost infinity) so, these 20MOhms are result of the input dividers. And what will be the impedance, if you connect one of the differential inputs to the ground, depends on the schematic. 

There are generally two variants, simplified shown on the below schematic. If you ground the negative input, the left schematic will have a half of the differential impedance - i.e. 10MOhm, but the right one will have the full 20MOhm.







simulate this circuit &ndash; Schematic created using CircuitLab