.....my 5 cents.... about JDM
May be take a look in rodeo or troopers manuals for 2wd-4Hauto-4Lo schematics. trooper (2001) utilizes vac control solenoids to move lock ring in front axle to connect/disengage driver side front shaft. on one of the troopers I have in the shop the TOD check light was flashing till I fix one of the vac control solenoids.(was not going to 4wd mode).
I saw the same problem being experienced with a Jeep something or other (don't remember what model it was) years ago, but in that case, some of the actual rubber/plastic vacuum lines had been damaged.