2003 F350 4x4 Front Hubs Won't Autolock

I just came back from a trip in the snow where I needed the use of 4 wheel drive. I was driving a 2003 F350 4x4 crew cab long bed 6.0 diesel I recently purchased. I turned the dash switch to 4 wheel high and assumed the transfer case engaged the drive shaft and the hubs (they were set to auto) engaged the front wheels. The snow on the road increased as I continued about 15 mph up a slight grade. It wasn't very long when the rear of the truck started to slide. I drove a few hundred yards to a safe spot, got out and set my hubs to manual. I got back in the truck and no longer had the sliding problem.

What can cause the auto hubs not to engage properly? Is there any test I can do to trouble shoot them? On one note I just had my ball joints replaced. The shop who did the work insisted that they replace some axle seals they said were part of the system that operates the auto lock hubs.

Any help is appreciated.

 

All sorts of things can cause this but the most common is a vacuum leak that is well documented in this forum. I went with manual hubs on my 08 because of the issues I had with the PVH system on a previous vehicle. The leak came from the hub and it was a nightmare to diagnose.