As the title says, on my '03 F-350 my 4x4 won't engage without the hubs locked. My shop told me it was the 4x4 vacuum switch and replaced it, but still isn't working. They also just replaced the front drive shaft which had worn out knuckles that were "grinding." Thoughts on the next step?

 

sounds like one of your vacuum lines is leaking,Maybe fell off or a break in the line.I took mine out completely and went with Warn locking hubs and don't have this problem any more, when I want 4X4 I just lock um in and presto I have 4X4.

 

Did you replace the O-rings on the hubs when you replaced drive shaft?
Welcome to guzzle's 4x4 Autolock Hub Lubrication Maintenance Web Page
from what Ive read that seems to be the main culprit for it not working, asuming you/they checked all the vacuum lines like psvienneau had said