First of all: Welcome to TN.
Second. There are a number of reasons for these differences in towing capacity. One could be that they had put some strengthining in the chassis after 1996, or that they had put a 'heavy towing kit' on there. I'm not too sure which way they go, but i know the heavy towing kit requires some suspension work. For example, my folks Falcon has the standard towing kit on there - which is 1.5t, but with the heavy kit, which has some chassis re-enforcement and suspension work is able to pull 2600kg.
So theres your answer as to how you can increase your towing capacity. Ask your local ARB Dealer... i think they actually made heavy towing kits for the 80 series LC's