4x4 lights on at meter panel

Hi there, can anyone tell what's wrong with my '96 Disco V8i when the 4x4 lights appears on the meter panel? It only happens when the car goes on long distances. I can only think of a few things that might happen :
1. The LSD lubricant is low
2. Something wrong with the transfer case
3. Faulty universal joints
4. Faulty sensors
Your expert and past experience to the above problem might help to solve my problem. Thanks a lot!