I have been having almost the exact same problem. I replaced the vacuum switch on the transfer case, vacuum lines, the vacuum actuator, cable, and axel actuator. The only thing left I can think of is replacing the short axels. I think the threads get stripped pretty quickly once any of those other parts start to fail if you don't catch it in time and are still trying to switch between 4hi/lo and 2hi. I just bought two new front axels so if that's not the issue after I install the new axels I'm going to go crazy!