I have a two amp 5VDC power rail. I have configured a motor shield to use external DC (by moving a jumper.) Is it ok to share the rail between the arduino's 5V and Gnd pins, and the motor shield?
the Vin pin on the Arduino is the source to the internal voltage regulator. It needs about 7 volts in order to work. If you try to feed 5V into the Vin then the Arduino won't get a full 5V.
Motors draw a lot of current, and cause transients on the power line as they switch on and off. (They will cause transients even in a regulated supply, because it takes the regulator time to respond to the rapidly changing load of the motor.) If you're going to feed both your motor shield and the Arduino's 5V from the same power supply, you will need a reservoir capacitor between 5V and ground as close to the Arduino 5V input as possible. The reservoir capacitor will even out drops and spikes in power to the Arduino.
You'd be much better off feeding 7-9V into the Arduino's Vin. That way it's power is regulated separately.