4kcsq center differential won't unlock!

[pfhamilton at ucdavis.edu]

I've got an '86 4kcsq with over 200k on the clock.  I've given it to my 
folks, who recently started complaining about its performance.  Turns out 
the center diffy is permanently locked (differential lock indicator light 
is on), and it won't turn off with the switch.  Obviously, this hurts tight 
cornering performance somewhat :-)  I think they've been driving it like 
this for a while -- hopefully this hasn't trashed the front drive!  
comments?

Anyway, I've gotta get it unlocked.  I've been under it for a little while 
searching for vacuum lines, as I assume one of them is leaking or has 
detached or something.  Man, it's tight down there.  I think the vacuum 
servo is between the battery box and the cat.  Don't want to start taking 
things apart until I know for sure.  Is the interior lock console switch 
electronic or vacuum?  Could this be a simple relay or wiring problem?

Help!!!

--Pete

P.S. please e-mail me at this account (pfhamilton at ucdavis.edu)