Holding torque is when the stepper isn't turning, so no amount of "holding torque" will move your axis.I am not sure how to calculate how much "Holding Torque" a motor would need to have in order to comfortably move my 2 pound Z-Axis around
How are you moving the Z axis (screw?) and how fast do you want to move it. You need to determine how much force you need at a given speed, and calculate how much torque you need to provide that force. Then you need to look at torque curves of motors, to find one to give the needed torque at the given rpm.
But in reality, with a screw drive, any 200 oz stepper should be able to move a 2lb Z axis with no trouble at all.