HELP my 83UrQ diffs are locked

["Dale Wheeler" <Dale.Wheeler@vacgen.com>]

Driving my 1983 Urq (non-torsen) with the diffs locked yesterday, went to 
un-lock them but only the front one un-locked, much tire-scraping since then 
round the corners. I'ts done this before and after a while the rear ones have 
unlocked, but this time it seems to have stuck permanantly !
 Anyone had this before or can offer any advice as to how the thing works ?, 
it's the vacuum-hose operated system.
TIA, Dale