Glad you figured it out and it was a free fix! My kids have kicked on 4x4 or auto without me realizing when playing in the truck while I wash or work on it, usually I notice as soon as I back out but I made it to the work parking lot once in the rain before I figured it out... I really wish there was an instrument panel indicator for auto4 or 4x4 because the steering wheel blocks the actual knob from my vision, and the indicator light is tiny anyway.
Because of this thread and 2004Slickside's post, I googled the auto vs 4x4 and came up with this video https://www.youtube.com/watch?v=YB9sGeAzoPE which explains auto vs 4x4, there might be changes in later model years but I learned much from this video. Thanks for that note slickside, because i didn't realize that auto was different than 2 wheel vs 4 wheel.