Changing The Dock's Appearance From 3D to 2D?

Yes, you can, but only in OS X 10.8 or earlier.

You can use the the following Terminal command:

defaults write com.apple.dock no-glass -boolean YES && killall -HUP Dock

To go back to 3D, run the following command (replace YES with NO):

defaults write com.apple.dock no-glass -boolean NO && killall -HUP Dock

The following is a script you can save as whatever you want. I call it DockChange.sh. Make sure it has the .sh extension on the end.

It needs to be chmod +x before you run it; so using the name above I would need to type chmod +x DockChange.sh into terminal, then to run it I only need to type ./DockChange.sh and it will do it's magic.

#!/bin/bash
dock2d3d=$(defaults read com.apple.dock | grep "no-glass")
if [ $dock2d3d == "\"no=glass\" = 0;" ]
then
  defaults write com.apple.dock no-glass -boolean YES && killall Dock
else
  defaults write com.apple.dock no-glass -boolean NO && killall Dock
fi

This script will automate the process for you.