Seems to be the most common problem, this was my disco the other month, and they had been serviced 6 month earlier
The pic is taken from the drivers seat, as you can see bar sitting through the piece with the shiney nut lever is the slider from diff lock, going down on the pic
Access is not easy from below, but a good soaking with spray oil may free off the linkage, it runs vertically from the box, you will see the sender from below, spray it up try to get a wire brush rounf the joints, and use the lever for a while back and forwards also up and down in hi lo range, dont force it though