2002 F-250 Auto locking 4 wheel DR. not workling.

I can manualy lock my hubs but I can't flip the switch inside to automaticly lock my hubs. When I go to automatic I can feel the transfer case engageing but it doesn't lock. Can it be a vaccum problem? Ray

Quite a common problem with these trucks.You have hub or hubs bad
or 4x4 vacuum seals leaking.Ford has updated washers to install when
installing vacuum seals to allow more vacuum to travel through.Its a
project to replace.