My main concern is what will happen if someone tilts my rail at an angle and then I have a 5 pound camera and lens fighting gravity. I don't want to rail to somehow drop current and fall back to the bottom and destroy someones gear.
This is a very strong argument for a gearbox. You may or may not use a stepper with it. The stepper alone would require battery draining holding torque. The Kessler mentioned is a servo system. It is using a DC motor, a gearbox and some sort of position sensing. That's a SERVO regardless of what they choose to call it.
The inexpensive and versatile 28BYJ-48 Gear Stepper Motor has been used in similar applications. I'm not sure if it is powerful enough for you.
You might look at this:http://forum.arduino.cc/index.php?PHPSESSID=ab125d03aadc7e498e8e2e5b27e544b0&topic=89159.0
The easy driver is $14 per one and the motor shield is only $19 so wouldn't it be better to go with the motor shield (assuming it's what I need to use)
You should be aware that stepper drivers such as EasyDriver provide considerable benefits including better torque, efficiency and smoothness of operation (less vibration).
What are your thoughts on using a threaded rod that runs through a nut that is attached to the plate that slides? The stepper motor would be connected to the rod and as the rod moves it would push the plate forward. I could greatly reduce the power to the motor in between motion and since it's a threaded rod I don't see the plate being able to slide back down the track unlike it would if it was attached with a belt and pulley system. Do you think this idea would help me solve my battery/holding torque issue?