Why measuring a differential signal requires a differential probe?
I am planning to measure an RS485 signal with an oscilloscope.


And...


  In my application, the transmitter circuit's reference ground(also the
  signal's ground) and...


Here's the contradiction. There are two basic signals in RS485 - they are differential and balanced and they need to be so. If they are not then it's not RS485. Then you say that the transmitter's reference ground is also the signal ground and that cannot be so for RS485.

If you are trying to measure a proper RS485 signal, you should not connect one of the transmit lines to the oscilloscope ground reference because that will load imbalance the signal (at best) or at worst short that signal out.

Use two single ended probes for each of the transmitted signals and subtract the signals using the oscilloscope features.