I'd just throw it in 4A, and drive normally until you could get a vacuum pump, and check things out. Driving it in 4A will lock in the IWEs and prevent the grinding. You've got a vacuum leak somewhere. If you were to take it into the dealer, the latest TSB, with a 4A, Torque on Demand Tcase, is to eliminate the vacuum system altogether by removing the source and plugging the lines. If you don't mind not being able to disconnect the IWEs, that's fine. Either you can do it, or the dealer will.
Likely wouldn't hurt if you were to pull the IWEs, inspect them, and the wheelbearing, for crap in them that will cause a problem down the line. I'm not sure, but the dealer may take it on themselves and do that.
What way you go is your choice, but if you're at all handy with tools, some of us can walk you thru it.

 

I'm about to take this one. Replaced a sensor to try and fix my problems, to no avail.

I'll be replacing the IWE and the hub bearing assembly.

OP, here are some great videos to watch to get a taste for what the job will entail


PM if you want some links for parts. Mine are on the way.

 

Well done @tland4

 

I had the exact same problem as the OP - 2015 Lariat.

Took it to my dealer service dept. Long story short the tech wrote that he took it for a ride and could not duplicate. I figured that because it was only happening to me at 65mph plus and no such roads around here.

Lucky for me the tech investigated anyway. He found the one IWE not retracting all the way when in 2WD due to rust. He replaced the IWE and cleaned up all the rust.

Next time - or before it happens again - I will have the TSB done that kills the IWE's and just leaves them engaged.

https://static.nhtsa.gov/odi/tsbs/20...81917-0001.pdf

 

I had to replace some vacuum hose on my 2012 that a squirrel chewed through. Once replaced, I had 4WD back and no issues.

 

My truck is a 2018 f150 xl regular v6 6 speed auto 4x4. bought it brand new 51miles on it in Houston ,TX May 30,2019 and drove home to Ohio. I currently have 5,300 miles on it. It just started the grinding noise befor Christmas. It is intermittant. I noticed it going up a long steep highway road yesterday. It goes away if I swith to 4 Hi. If I switch back to 2 Hi it comes back. On a level surface it will start to grind but if I leave off accelerator it stops. Seems vacuum related to me. Plan on calling another Ford dealer Monday. Still have 18 months warranty and 30 K miles.

 

Ya, I agree vacuum problem. I usually suggest replacing the checkvalve as it's cheap and easy. Until you do, or get to a dealer that might fix it, just pull the power to the solenoid, IWEs will lock in, and prevent any more grinding.
Tube Assembly - Ford (FL3Z-3A788-B) | TascaParts.com (1)

 

Hey John, will pulling the power to the solenoid also engage the 4x4?

 

No, all it will do is lockin the IWE. You still need to shift into 4A/4H/4L as you normally would.
If a guy has 4A, you could also just shift into that without pulling the power. Ford says you can run in 4A in all road conditions. Some guys here just run 4A throughout the winter.

 

I just had a problem with my IWE and it was a faulty check value. Quick fix, maybe worth a shot.