Differential and single-ended input impedance
It's a bit unusual to hear single-ended and differential. Are you sure you don't mean common-mode and differential?
Differential is measured from one input to the other input. (It's what a signal sees if it flows into one input and out the other input).
Common mode is measured by shorting the two inputs together and measuring between that node and ground. (It's what a signal sees when it appears on both inputs).
Single ended would mean between one input and GND, but that is a meaningless measurement for this kind of circuit since you don't send a signal into this circuit that way.