Mine was because my vacuum lines between the hubs were bad. I got the clicking and flashing to stop, but now I think I may have a bum hub cause I have the 4 hi and 4 lo light working, but not actually driving the front wheels. I am going to replace the entire line first and make sure the spongyness isnt what is kicking it out. I can get power on flat level surface, but as soon as I incline the fronts stop.
Hope it helps.