2002 Mitsubishi Shogun flashing differential light

When 2 wheel drive is engaged the green differential light flashes and after approximately 10 minutes of driving, when the car is brought to a standstill the yellow ABS light is also illuminated and stays on until the engine is turned off. When the ABS light is on the ABS is disengaged. It does seem to change between 4WD and 2WD but you have to reverse to do so (sometimes a couple of times). It has had the switch that controls the change renewed but this hasn't resolved the problem. The ABS has also been stripped and tested and there are no problems there either. I was told that the locking wheel hubs(?) operate on a vacuum and wondered if this was part of the problem although my garage has checked these are reports no problems there....

Can anyone suggest a solution?