If it has the same lever mechanism as my disco (which I believe it has) the the cross sliding mechanism on top of the box seizing from lack of use is probably your culprit.
The shaft from the diff lock lever is fixed to the linkage by a pinch bolt over a splined joint, with this released you will be able to see if the lever is stuck. If it is, you need to remove the four bolts to get it off the top of the box. You should be able to free it up without to much effort with it of the car. I made sure it was smothered in coppaslip before reassembly.