The standard Land Rover thrust washer is 3mm thick, when these wear down, the high range gear clearance is increased between high range gear and Input (middle) gear, this allows high range to jump out of gear under load which may lead to further damage to the dog teeth engagement into the intermediate input gear. Maxidrive used to manufacture a thrust washer set to allow the High range gear to move closer to the intermediate gear by 1 mm, this set consisted of 4 washers: 1 @ 2mm thick, 2 @ 3mm thick, and 1 @ 4mm thick.
3mm went either side of the low range gear, as per usual
2mm went between the high range and input gear
4mm went between high range and casing
The effect of this allowed the high range gear to engage into the input gear by a further 1mm.
Unfortunately, this kit is no longer available. All one can do these days is replace the washers with standard set, but....if you use the thinnest 'worn' washer between high & input, and then make up for this clearance by using a thicker endfloat thrust washer (3.55/3.63/3.74mm part no. 591899/591897/591898) you may be able to improve high range engagement.
