Transfer Case?

I'll assume you mean engaging the low box.

The RRs at that age should have a Borg Warner transfer box. IME these are much easier to use/change ratios than the LT230 fitted to a Disco. The easy way to spot the BW box is the lack of a diff lock position on the lever as the centre diff is permanently locked in effect.

The LT230 can be a pig to get to go into low box, generally I change on the move - put the gaerbox into neutral (you can dip the clutch in a manual) and at LOW speed move the transfer box into neutral then put it into low. Put the box into drive and start driving then give the lever an extra push into low to make sure it's fully home.

If this still jumps out then it's possible that the linkage needs adjusting - they can be a bit tetchy when you only have a relatively small amount of lever movement as in the Disco.

cheers

Dave W.