You're in the MY area where LR say that the the linkage was taken out, so a major job and cost to refit the parts, although apparently a kit was available to do the job.
Feel around the top of the transfer box at the front and see if there is a short shaft poking up out of the top of the box, if you have that, then you have the linkage, if not then you are not able to hook it up.
Pictures below. Note that the lever for the diff lock (No 39) is present from MY 03 onwards, but missing on the earlier one.
Have a look, if you have the shaft sticking out, then you'll need the Diff Lock linkage from a Disco 1 or try and buy the bits. I think that although LR said it was removed internally, you'll probably find it is in there.
Peter