Four Wheel Drive Problem- 4HI

I own a 1997 F-250 7.3L powerstroke turbo diesel with 303,000 miles. The truck runs and drives great. Ibut i'm having trouble with my 4hi. I try to shift from 2hi to 4hi and it just goes right past and slips into nuetral. My 4 low works fine but if I do get it into 4hi I just can't get it to stay in and it pops out. Both front hubs are fine. I took apart my transfer case and everyting looks pretty good, just a little normal wear. The transfer case is a borg and warner 4407 and i'm having trouble finding info and diagrams of it. If anyone could help me out I would appreciate it.