I own a 2003 f350 7.3. when i first bough the 4x4 was not working. i took it to a shop and got it fixed. i even got to use it once to get out of some mud.. any who, the auto lock feature never worked, which is not much of an issue for me, i don't mind getting off and manually locking the hubs, which i did that day. i could have swore i unlocked them when i was done, which obviously i didn't because i ended up going on an out of town trip a few days later and i was doing about 80mph... over 300 miles later, the truck started shaking really bad, i got down and made sure a tire wasn't flat or bad and they weren't, i went another 5 miles and did it again... i finally checked the hubs and they were in the locked position... so i unlocked them and the truck ran fine after that. any who... when i got to my destination i tired using my 4 wheel drive and it worked, but it is making a really bad clanking noise in the front end. sounds like something is about to break.. (only does it when the hubs are locked) I'm guessing the only thing that might have gone bad are the hubs, but i want to be 100% sure before i buy them. is there anything else i should check on before buying the hubs?

 

Look at the U-joints on the front drive shafts. The drive shafts should only be spinning if the hubs are locked in or if your transfer case is in 4wd, so it would explain why you only hear it in 4wd. While you are down there I suggest checking the wheel bearings to make sure they are good, because, they can go bad as well.

I had my right front u-joint out, and while my wheel bearing didn't make any clanking noise (that was the u-joint) it was bad, and I have a feeling it "helped" the u-joint live a shorter life.

Baatzy

 

Like Baatzy said, check the u-joints, particularly on the front driveshaft.