1996 ford limited control trac

problem with 4x4 hi and lo dash lite flashing on & off after selecting 2 hi. Shift motor has been replaced. Snow storm 3/20 got along ok till later in day i had backed up started forward and front wheels would not pull, stuck on ice & snow this happened the next day again. Seems like if don't back up front end stays engaged. Anything with the vaccum- electric setup on the front axel??