Okay this is what I got; I recently purchased high mileage 2004 F250 Super Duty short box super cab 5.4 liter. Came with manual 4x4 handle on the floor so no 4 wheel on the fly and this eliminates the vacuum for the auto hubs so this will not be part of the problem. When engaging the hubs and ONLY when engaged the driver side hub grinds. The 4 wheel drive works but it sounds like its ready to grind itself into oblivion. I did pull the hub and the grinding was gone, I happen to have a spare hub that is good and the grinding returned. I have jacked it up and made sure I have no play in the wheel itself and the axle knuckle on the axle shaft. Some say remove the front drive shaft? Or turn the wheels all the way in one direction engage the hubs and check for binding? I will do this in the morning, but what does that confirm if it not the hub. Could it be the passenger side echoing across? On my 99 Super duty I kept hearing moaning coming from up front like the breaks were shot and if you stuck your head out the window it was the back echoing to make it sound like the front. That's a different story so anyway any suggestions?

Thanks!

 

I could be wrong and I know you said you replaced one locking hub with a good working hub but I would tear both down and clean them and re pack with grease then try it again. The only other place is the spindle or driveshaft knuckle that I can think of to give you a problem.

 

Its the knuckle I can see it twist. Looked good in the dark, but after jacking it up and turning the wheel in one direction and trying to turn it I can see the knuckle twist and the tire is very reluctant to turn if it is in one direction. Ill replace and update.

 

I do not know if it was a multitude of worn pieces or not. After replacing the knuckle I got the same grind. I jacked it up on 4 jack stands and put it in drive with the 4x4 engaged. With no load I got no grinding but a small clunk and by watching the front drive shaft rotate abnormally I pulled it out and the pivot ball closest to the transfer case was shot. Rebuilt; problem solved!