Ford Super Duty (1999-2016) :: 2008 - Message Flashing - 4x4 Low?

I just traded in my trusty, never had a single issue 2008 Ford F150 with 53k miles for a 2008 Ford F350 with 112k miles. I figured the extra miles were okay, because it is a diesel. I purchased the truck to pull a 31’ 12k lbs when loaded toyhauler, and I have a 14K weight distribution hitch.

Just in case this makes a difference, here is some background information. I have a 2008 Ford F350 XLT 6.4L CrewCab Long Bed. Before purchasing it, I had Ford do a performance and safety inspection. Everything came back clear, so I bought it. The day after purchasing it, I changed all the fluids for peace of mind, ie. rear diff, transfer case, transmission, motor oil & filter, fuel filter, and radiator fluid.

Here is the problem. Occasionally, the 4x4 Low light will start blinking/flashing. Sometimes it will only flash once or twice and go off, other times it will flash for 15-20 seconds and then go off. There is no pattern to when it happens other than it seems to do it mostly when slowing down or breaking. It is very sporadic. I don’t drive very far for work, so I can go an entire week without seeing the light come on, but if I hit the highway for 30-45 minutes, the light will eventually turn on.

When it flashes, occasionally, I get an RPM increase…almost like it’s trying to engage natural or shift into 4x4 low at highway speeds…which causes me concerns. I took it to Ford and after 50 miles of driving it around, they could not get the 4x4 low light to come on. They said they pulled codes but there were none to pull. They reflashed the PCM and charged me $99 to not fix my problem.

I can shift into 4x4 high and 4x4 low (with the window rolled down, I can hear the shifting from 2wd to 4wd high and then to 4wd low and back to 2wd), so I don’t think it is a blown fuse or the control module. I removed and cleaned the wire connectors to the shift motor on the transfer case, and that didn’t fix it. I was thinking about replacing the rear diff abs speed sensor in hopes that will fix it. I think that maybe it’s a wiring issue, which I wouldn’t even know where to start.

I was getting 14 mpg before the reflash and after, I am now getting 8 mpg. Does this seem right? Or is there a learning period after a reflash before my mpg gets back to normal.

Also, the automatic 4x4 drive doesn't work. I don't think its holding a vaccum. I don't think it is related to the 4x4 low light coming on when I am doing normal driving. I can use the lockers to get 4x4 to work correctly. Infact, Ford is the one that told me the auto doesn't work.

Edit: I also noticed that if I flip ON any of the upfitter switches (ie AUX 1, AUX 2, AUX 3, AUX 4) the 4x4 low light will flash constantly until turned off. I currently don't use any of the switches for anything.