06 F250 5.4 4x4 Won't Unlock
 

Supercab Auto
Had this issue on/off for a while, and year ago it seemed to unlock itself after a few tries, then no issues for a very long time, then 2 months ago it locked in 4x4 Low, so had mechanic look at it and he replaced switching motor, and that appeared to work for a bit. Then 3 weeks ago it locked in 4x4 Hi, but it unlocked 1st thing next morning. Then no issue for few weeks, and I use 4x4 few times a week. This past weekend it locked in 4x4 Hi, and wouldn't come out, but I manually switched hubs (explained below) and at highway speed for 5 or so minutes at 50 mph, i switched it out to 2wd (thought heat possibly). Then next day when i used it, it locked and now it won't come out again.
Background, the 4x4 fender switch i assume is failed, cause I have to manually switch hubs, and theres a 4x4 solenoid error code P1876, but 4x4 has worked fine for couple years. Other issue, not sure it's related, but the fuel tank level fails once in while, shows low fuel error B1201, but fuel pump is fine and I haven't traced that level issue.
For the 4x4 switching, appreciate any ideas.

So had the electronic switch motor replaced, then it switched back and forth to/from 4x4 fine for about a week, then tonight it locked up again. The only thing I did since the last time it worked fine was power wash the truck. Isn't the switch motor supposed to be waterproof? I'll check connections again, but appreciate if anyone has seen similar issues. I do wash my truck quite often, as I'm often on a dirt road that muds up.

So in looking at a related thread, saw a video link for '97 Intermittent No 4wd...", that showed an issue with a behind the dash relay (7-point?). Does my 06 F250 have a similar piece?

Turned out to be the Transfer Case, shifter extremely hard to turn by wrench with the shift motor removed, so replaced it, no problems now, and ordered the Dorman vacuum shift solenoid for front hubs, just have to vacuum test and swap out the old solenoid