200 F150 5.4L Triton, 164K rounds. My odometer has been going on and off intermittently since I bought the truck 4 months ago, no big deal. This morning, though, I started the truck to go to work and my oil pressure gauge was at zero. There wasn't any noise coming from the motor aside from the very slight bottom end knock it had when I bought it, so I figured it's the sending unit. I shut the truck off and started it back up a few times to be sure, and the oil pressure sending unit started working. But THEN my 4X4 engaged. The dial on the dash was in the 2WD position, but the 4X4 was definitely engaged. It wouldn't engage when I first bought the truck without swapping the vacuum hoses under the hood, and now it's doing it on its' own. Any thoughts?

 

Seems you might have two issues at the same time.
The dash electronics circuit board need some attention with a small soldering iron or the connectors need reseating and the GEM may have corrosion from getting wet to often.
It would be good to know how 4x4 works in order to see what may be an issue.
The dash lamp coming on normally happens when the case motor has rotated and the firewall solenoid operates to normally indicate successful engagement.
In your case it likely is an issue with the GEM causing uncontrolled operation.
The GEM is located on the rear of the dash fuse panel.
Good luck.