03 f150 4low works but 4 high doesn't
 

Hey I have an 03 f150 and for some reason my 4low works but my 4 high doesn't. The light on the dash comes on for both. I took it to a dealership they said it might be the shift fork is broken but that doesn't add up in my mind. From what I can tell it has vacuum. I have tried searching online but I haven't found an answer so maybe one of you guys has an answer