All the transfer boxes fitted to LR 90, 110, RR and Discovery are mechanically interchangable. They may, however have different ratios.
IMHO... avoid fitting a viscous-coupled box (aka a Borg & Warner or Chain Drive box)... they are too prone to failure if from an ageing vehicle.
From memory... the transfer box ratios are as follows :
LR 90 : 1.4:1
LR 110 : 1.6:1
RR Classic : 1.2:1
Disco 1 : 1.2:1
Changing the box to a different ratio will have the effect of changing the overall gearing of the vehicle in High Ratio. (Low box ratios are the same for all)
Hope this helps.