The linkage slipper is on the main linkage from your 4wd handle to the t-case linkage rod. Its adjustable to let you move your 4wd lever, to a comfortable position for you to grab. However if it works loose(common), or youhave it adjusted too far in one direction or another, you will lose the positions on 1 end of the sweep.
i.e., if its out too far, you can get neutral and 4low, and if its in too far, you can get 2wd and 4wd.
Look on the main shaft coming off of your 4wd shifter, you are looking for a small section of c-channel with a bolt screwed into it, the bolt is the locking mechanism.