Don't forget there is not really a true default position for the diff lock/decoupler. The vacuum actuator at the diff lock or decupler is pulled in each direction with vacuum selected by the switch position.
If you think of all the likely failure modes, two common ones are a stuck actuator shaft or loss of vacuum (at the actuator), each of these failures will result in it failing in the position it was last at.
In the case of a stuck selector shaft, it's most likely to get stuck in the position it spends most of it's time (eg off), with a loss of vacuum, it depends on where the leak is, if it's on the "supply" line to your dash switch, it will fail where you were last, if it leaks between the dash switch and the actuator, it will not switch in the direction of the leak. (I highlight "supply" as you don't supply vacuum, it just opens the selected line to vacuum from the engine)
If you wanted, you could also swap the vacuum lines at your actuator, this will swap which direction the actuator pulls in relation to the switch position. It doesn't achieve much as there is not really a default position, also this would get confusing though as your switch would be pulled out for 2WD and the light off.
In a break down situation you could just remove the vacuum actuator and manually put the selector shaft in the desired position (well not easily as you would need to align the drive dogs some how while pulling the selector)
Another interesting point to note is the addition of a reservoir and a check valve (between the engine and vacuum reservoir) in the system. Your engine manifold vacuum fluctuates depending on engine load, so if you left these two items out of the system, the vacuum at the actuator would also fluctuate during normal driving.
If you think of your system having a very minor leak, leave these two items out (check and res) and your driving under heavy load for a while (low engine vacuum) , if the system had a "default" position you risk this changing inadvertently.
I would consider the system safer as is rather than having a default position.
http://www.syncro.org/sitephotos/decouple.JPG
http://www.obsolyte.com/vanagon/VanagonProTraining/86Vanagon/040.jpg
Richard