Transfer Box and Diff Help

New to this Guys! 1991 Defender 110

Transfer box. Seems all to work fine in high, but selecting low or diff seems all pretty unsure. Occasionally seems to get in then jumps out. Any idea how I can tell whether this is linkage or transfer box problems> Anyone got the actual driver instructions for how to use this??

ok what you need is a can of WD40 and from under the 110 soak all the linkages and rods and leavers on the gearbox and then try again