I think the 1983 standard factory set up is with the LT95 transmission.
There is a hi-lo transfer gear lever and a seperate centre-diff lock plunger that sits in the center of the seat box. It plunges forwards and backwards.
The centre-diff lock is air /vacuum activated - so there are chances for leaks and otehr stuck linkages.
The Defenders went to a different configuration with the LT77 and the R-330 transfer cases later on. On these gear boxes, the diff lock was engagued from the hi-lo lever.
Firemanshort
1980 Stage One
(Past owner of 1973 Series III - Highlander)