Differential signaling is a method to provide noise immunity. For instance when using twisted pair in an ethernet, the idea is that outside noise will affect the signals traveling on the + and - wires more or less equally. So assuming you intend to send a signal A0 across the cable, you convert A0 to +A0 and -A0 and push them down independent cables which are twisted together. The cable is in a noisy environment and picks up some noise which I'll call e. In reality, both cables would not pickup exactly the same amount of noise, but we can ignore that for this discussion. As a result, we have two signals that are now the sum of each original signal and the noise component epsilon: +A0+e, -A0+e. At the receiver we now take the difference of the two signals, delta. D = +A0+e - (-A0+e) = 2A0 = A0.
RS232 is not differential signalling however, though the voltages swing from -12,+12, the transmit is only a single wire, as such, you cannot use the noise elimination technique I just mentioned. RS422 and RS485 (for which drivers are readily available) are similar to RS232 using differential signalling.
Two meters is not very far. It is very far for a high speed signalling system, but assuming they are using the PICs UART, and you can live with the lower data rates, you could use a single ended line driver or maybe get away without a line driver at all. As the speed gets higher, you get lots of attenuation from the cable impedance. For comparison purposes, a regular 100BT ethernet runs around 125MHz or so.