The 4 wheel drive usually becomes inoperable because the car spends all its life in 2L.I had the problem with my LWB and the diff lock wouldnt engage .Mine was a wide bodied UK one and it does say in 2 or three places on big stickers to operate the full 4 wheel drive so it does not become seized iirc and yes it is vacuum operated .

http://www.pocuk.com/ this site helped me no end all by using the search,I never even had to join .