Ah, I see. Okay, so it's that you are using AC in the signal processing sense that you are not driving the motors with a purely DC signal. You are pulsing a PWM from a DC source. Yeah, everything I said before can still apply.
EXCEPT you will have to IC-sense chip to convert the voltage read across the current resistor into a ground referenced voltage (in ALL cases regardless of whether you go low or high side current resistor) because the way your circuit is set up, the same current resistor will alternate between high-side and low-side depending on which MOSFETs are on. Also, your ADC will be destroyed if you connect it directly to measure the resistor voltage because it can't handle the negative voltage across the current resistor because the current can travel through the resistor in both directions (assuming you place the resistors in all three T-intersection, rather than than at each ground terminal...for symmetry sakes.
If you place it at each ground terminal, then the above doesn't matter.
About the milling machine, I think they all use steppers because the milling motors move really REALLY slow and steppers move best at slow speeds. To use a DC-servo motor, it would take a serious amount of gearing to move motors of that size at that speed, or you would need ginormous motors (and still have gearing, but less).