There should be plenty of MTN cranks in 170mm length although I'm not sure if they make MTN specific cranks in the 172.5mm length. I'm not familiar with those specific cranks although the first question is what interface do they use? Are they Octalink, square taper or outboard bearing?

What measurements have you used to decide that you need just a 2.5mm shorter crank? 170mm or even shorter may be better. What's your actual inseam?

Anthony