Wow, lots of action in here. Thanks guys!

Power supply is at 48VDC. I'll check tonight to see if voltage drops under load. It's a toroidal power supply (part number: KL-4820)

It's not clear to me if this closed loop stepper implementation can take advantage of the PPR of the quadrature wave - essentially turning your 1000 points per revolution into 4000 discrete points. It eludes to this, but doesn't say it explicitly.
Yes I believe it does. PPR is set to 4000 from the factory.

Things to consider:
Are you wired up correctly? I believe so, they work just peachy without any load, and they come prewired from the factory. All I did was extend the leads on them and add a connector to plug into my enclosure

What kind of amperage is your power supply? It looks like these things can handle up to 6amps. 20A max could I be pushing too much current thru them? Hope I didn't burn anything out!

Do you know your actual torque requirements for your system? A holding torque of 2 Nm (~282oz-in) really is about 70-80% of that in the very low RPM range, then will continue to decrease. Really you're probably at around 225-200oz-in torque in the 100RPM range. I did not, but I use smaller steppers on my other mini mill, and they have WAY more torque at 48V than these buggers do.

What RPM are you running your steppers at? I've tried varying the RPM with no real difference. I've gone down to 10RPM up to around 200 RPM with no noticable difference