An ungrounded differential signal source can float outside the common mode range of the logger inputs. Possible ways to prevent this are:

grounding one of the signal lines at the logger
grounding one of the signal lines at the logger through a moderate value resistor, eg. 10k (http://www.ni.com/tutorial/7113/en/)
grounding both of the signal lines through high value resistors, eg. 100k, 1M, 10M depending on the output impedance of the source (http://www.ni.com/white-paper/3344/en/)

It would seem that the high input impedance of the loggers would easily allow the a differential signal to "float" out of range.

Should one of these methods be used with Datataker loggers?

Good evening Anton

Depend on how much the differential inputs are floating.
The DT80 range can have differantial input grounds floating to +/- 100VDC from the supply Voltage ground.

Cheers Roger