Can an ultrasonic sensor differentiate b/w objects?
No. No, you can't. All they sense is the "time of flight".

The closest you would come would be to have a sensor which can detect physical properties of the target.  That could be colour, sound absorption, etc.  You'd need something that sends out a "broad spectrum" signal (white light, white noise, etc) and detects the reflected spectrum (colour sensor, wide-band microphone) and do spectral analysis of it.  The signal would also have to be fairly tightly focussed to exclude signals from the environment.

All-in-all not something for a little Arduino to consider without lots of extra expensive hardware.