I would like the option to have 2 wheel drive low range for backing the trailer. If I pull the number 39 fuse (4x4 hub lock) will the transfer case still shift to low range but the auto hubs not engage?
My last super duty (2002) I just used a golf tee to plug a vacuum line but this 2015 model is a little different.
I also read somewhere to put a toggle switch on the red wire going to the pulse vacuum hub lock but I think pulling a fuse would be the easiest way.
Any suggestions?

 

It would be best to put the model/year of your truck in your signature to help people assist you so we know exactly what we are talking about and provide more accurate advice.

The "Constant Vacuum Hub Lock Solenoid" is actually conveniently mounted on the back of your primary coolant degas bottle. It would be simpler to just disconnect it than pull the fuse in my opinion. If you need to do this a lot then you could also trick this out if you have upfitter switches. You can use a relay to open the circuit when you need to disable it simply by turning on a switch on your dash. This will likely leave a code in the transfer case control module but you can ignore it since you know the circuit was modified and you can ignore it.

Just to satisfy my curiosity, what is the problem with having the front axles engaged when maneuvering a trailer? Axle wind-up would be my only guess.

 

Most likely this ^^^^. I always have to use low range when backing my trailer into camp or its spot at on the side of the house since my truck is a manual and there would be way too much clutch slipping involved otherwise.

Fortunately my hubs are purely manual, but it would be nice on a newer truck to rig an upfitter switch to engage the hubs, since, where I park at home, I do have to get out and lock the hubs once I get off the road, as I have to back up a steep gravel driveway and the truck just spins out if it's not in 4wd.

 

 

Been there - done that .......
Take a look.

https://www.ford-trucks.com/forums/1...king-hubs.html