This won't work - the motor shield cannot supply enough current, and is designed for a higher voltage than
the stepper motor windings - its the wrong motor controller for a low-impedance bipolar stepper, these
require a chopper-drive circuit - and you really need a microstepping driver or it'll vibrate/resonate badly.
A motor rated at 2.8A is beyond the range a single chip driver can manage really, the best I've seen is
the TB6600 which has 0.4 ohms total on resistance, and seems to be available on eBay cheaply: http://www.ebay.co.uk/itm/TB6600-5A-Single-Axis-5A-CNC-Stepper-Driver-For-CNC-Milling-Machine-DIY-Max-50V-/300909044677?pt=UK_ToysGames_RadioControlled_JN&hash=item460f93a3c5
You could try driving it at a lower current (1A?) using something like an A4988 chopper driver board (Polulu
have some), but its a waste of a powerful motor.
Since these chopper drivers do automatic buck-conversion you power them at a high voltage like 24V -- 80V
and get far faster max step rates from the motor.
If you don't want fast performance its easier to buy a unipolar stepper with high-impedance windings (12V 0.5A
or some such). low-impedance bipolar motors are designed to go fast from a chopper drive circuit.