Use transistor to connect terminating resistor for differential signal?
I'm not going to explain why is this a bad practice, but rather mention why a single transistor won't do the job.



Say you have a setup like the following:







simulate this circuit &ndash; Schematic created using CircuitLab

Obviously, you need two FETs simply to tackle the fact, that reverse-biasing the Source-Drain junction triggers the device's intrinsic diode. Second, you'd have to make sure, that at all times, your enable would be higher/lower than your maximum/minimum single-ended voltage + \$V_{th}\$ of the FETs.

Say if your signals swing from 1-2 Volts single-ended, then your Enable signal would have to go from 1V to at least \$2V+V_{th}\$ (in respect to ground).