4 wheel drive problem
 

I would appreciate any input On the problem I am having with my 2002 ford F350 7.3 My 4 wheel drive doesn't seem to be working correctly. when I activate the switch on the dash the truck sometimes responds like the 4 wheel drive is engaged but then it doesn't, the 4x4 indicator light remains on even though it seems like the truck isn't in 4 wheel drive. when I switch back and forth trying to get it to work. It will work for a short period of time sometimes then stop working again. I have left the Hubs in the locked position for quite a long time and only relied on moving the switch when 4 wheel drive was required. The people on this site have helped me a great deal in the past so I figured I would ask.. Help! more snow is on the way..