Nema 23 Stepper motor not very strong?

I'm building what I hope will be a fairly heavy duty CNC machine. I've purchased these Nema 23 stepper motors. I'm using grbl on an arduino uno and powering the steppers using these drivers.

The problem I'm seeing is that even though I set grbl to never disable the steppers, I am able to turn the motor over by hand when the machine is on and should be holding its place. Furthermore, if the machine is moving I can block the machine with my hand and without much force, cause what sounds like a "buzzing" noise. I've taken the motor off my machine and done the same things by hand.

I really shouldn't be able to turn this motor by hand when it's engaged, should I? It takes some force and it moves in jumps when I do it, but it _moves_.

The motors are rated at 2.8amps. I've configured the driver to use 2.84A (peak). I'm not sure what "RMS" stands for, but RMS is shown as 2.03A. I've also tried jumping to 3.31A Peak/2.36A RMS with the same results. I'm currently configured with 8x microstepping. (I haven't tried different microstepping settings yet.) I've tried all of the above at different speeds.

I'm hoping someone here might have some suggestions on what the problem might be or somewhere else I could go ask for help.

Thanks!