This has happened twice to my father now.

2012 F150 less than a year old 4x4 will not disengage on several occasions, he would call the dealer and myself (I'm a heavy duty mechanic, work on this stuff all the time). Would use 4x4 around the ranch, get out on the highway and sometimes experience a substantial "CLUNK" when it finally let go, both the dealer and myself said we would look at it when he brought it in (he had the local dealer in the city he traveled to do a quick check and they figured he just hit something on the highway). The dealer he bought it from checked it and test drove it and said it was fine, he brought it to me, I checked it, put it through some tests and it checked out just fine.
Anyway a few weeks later he's headed to town again after using 4x4 and again it seems like it's locked up and this time it starts stinking, he stops, calls me, calls the dealer and explains the situation, the truck actually disengages while he's on the phone with me and I'm having him drive in circles while backing up and going forward, again a loud clunk. The rest of the day it's fine, takes it to the local dealer and they replace the whole front axle, it's in the shop a month while he's racking up miles on a loaner. He gets it back and less than two weeks later he uses 4x4 to pull a load of cows out of a muddy field, switches to 2H and 50km into the drive to the farm a trucker pulls him over because there's smoke rolling out the back of his unit, open the hood and there's a lively fire burning on the left side of the engine. Get it towed back to the dealer, it's in the shop for four months this time, gets another front axle and transfer case but this time he doesn't take it home, trades into a new 2014.

Fast forward to 2017 and 110,00km later he's checking some cattle in Montana and I get a call it won't shift out of 4L, they fool around with it a bit, drive thirty km till they get into cell service, go through the routines I suggest and finally it shifts, then it's good to last night...
I get a call at 7pm and he states he's had a fire, exactly the same scenario as last time, four head of big cows in a stock trailer, come off a muddy field at the ranch, shifted to 2H and gets pulled over because smoke pouring out the back, has to put the fire out on the left side of the engine, exactly the same location on the highway. As I'm traveling to him he starts it and drives into a transformer station and as he's turning it around he feels the clunk as it shifts out of 4x4. I check it over and drive it the remaining 50km home without incident but the front diff is definitely out as It was hot and I can lift the input shaft up and down a half inch.

I have a few questions...

1. I've driven a few 4x4s down the road several km locked up just for fun as a kid and for diagnostics later on (and maybe forgot a time or two ) and have never had an issue with hot diffs. Why are these not standing up? There is no way these front axles should be failing just because it is driven down the road locked up, if that's the case the owner's manual should state such and if it doesn't Ford should be liable for these incidents.

2. Anybody ever check if this may be the cause of some of these pickup fires?

3. Is there anybody good at Ford Canada I could contact to discuss this problem with?

4. And of course why are these transfer cases not disengaging? The shift collar teeth must not be cut properly? Or the shift motor is hanging up? Indicator shows it has shifted.

Thank you.

 

Check the gear ratios? Same front and back? Only thing I can think of and you're the first poster I've seen with this issue, so I don't know how much help you'll get from the forum.

Edit. Are the tire overall ratios the same?

 

Gear ratios are the same, tires all the same size make and model.

With the trailer loaded the radius of the rear tires would have been slightly smaller causing the front end to pull the whole unit but I think it should be able to handle it, get hot maybe but not fail so quick. It should have also disengaged itself when the truck turned the three ninety degree turns at intersections.

I talked to the mechanic at the local dealership today about the situation and he says these are the only trucks he's seen this happen on, Ford is baffled also. The powertrain warranty is five years 100,000km they say so they are going to contact Ford and see what they say, it's sitting at 113,000km.
He did say while up on the hoist the transfer case wouldn't disengage while the wheels were turning, they had to be stopped which doesn't make sense to me. It seems the shift actuator couldn't pull the shift collar while in motion, he said Ford didn't seem concerned with that.

 

The engagement mechanisms are STRONG! Crazy strong. Like break your arm like a twig strong.

The disengagement is just mechanical linkage and hydraulic pressure. I can't see a problem there unless it's a manufacture error.

It also doesn't address the overheat issue in the front pumpkin. It is the front diff yes?

 

The actuator is just a little electric motor running a spring loaded arm through a little gearbox. It wouldn't put five pounds force on the shift collar or it would burn up in less then a minute. You should be able to force it over with your finger no problem.

I'm not sure where you would get the idea it's some powerful hydraulic mechanism?

Yes it's the front diff.