RS485 is a symetrical two wire protocol. the same system as sound cables with the 3 pin XLR.
we often use this over two wire but these wire must be twisted for such a long distance. if your cable has a shield, ground it on both sides, this gives better noise to signal ratio.
anyway , most devices (modems) have a power supply without ground (only 2 pins on the 220v side) so the ground potential difference is not a problem. most 485 devices can run on one pair (half duplex) or two pairs (full duplex) if available use two pairs but if you dont have massive amount of data, one pair is ok.
in theory the same data signal goes over both lines, one signal positive, one signal negative. any distortion signal which comes to the cable adds the same peak (for example positive distortion peak) to both lines.
the circuit on the receiver side takes the postive signal from one line and INVERTS the negative signal from the other line. the distortion comes on both lines positive, but since the signal on the second line will be inverted, the distortion signal will be zero.
my installation with rs485 uses only one pair (taking data from a clock to a point aprox. 500 meters away) works perfect.
EIA-485 says that using the ground can be a advantage for longer lines, but its not nessecary.
wikipedia gives a few good explanations about differential or symetrical transmissions and EIA-485