Something else that may work is something we use at work called a Flag Switch I believe. It is a switch with a 5" or so spring instead of the toggle. If you could mount it so it just touches the shifter in 2wd then when the shifter moves it trips the switch. Could be rigged up to a light somewhere easy to see. We use them to indicate when a dump box is up on a truck, or older heavy equipment without a back up alarm. May be a little redneck for some, but sometimes it's the only option.
