Why is my truck moving(rolls back apparently)

I have a 2007 Ranger I use as a personal vehicle. It is a 4.0L 4x4 sport. It is an automatic with push button 4 wheel drive. 170kmiles

My drive way is about 30 feet long and slopes slightly towards the street. It is maybe 1.5 feet higher at the end than the street. It is mostly coated with ice except for where I normally park the truck. I park it at the non-street end of the driveway.

This morning I went out to my truck and found it was ten feet closer to the street then I normally park it. There was a 1/4 inch of snow overnight. There appeared to be tire tracks made from it being moved back in that snow. I thought maybe my sister who is staying with me moved it for some reason and went about my day.

When I came home I parked it where I normally do. I just took my dog out and found my truck in a similar position. I tried pushing it back further and could not get it to move. I hoped in and went to pull it forward and spun the tires on the ice, so I put it in 4-wheel drive and pulled it forward no problem. I left the 4-Wheel drive on and put it in park. I also put the E-brake on just in case.

My assumption is it’s rolling rather than sliding since all 4 wheels are on bare asphalt in it’s normal parking spot. I’ve never had an automatic roll on me while in park.

If I have time tomorrow I’ll drop it at my mechanic to check it out, but in the meantime, what the hell is going on here?
Not actually going into park?
Gremlins are screwing with me?

Also when this truck is turned off in 4-wheel drive does the it stay in 4-wheel drive or will it default turn off then turn back on when it’s powered up again?

Parking pawlbroken?
It’s always good practice to set the emergency brake.

1" inch… its just freezing , not far below, and there’s some snow melting too , right ?

So the snow near the truck might melt in the sun, but under the tire, its colder… Its below freezing under the tyre, since the tyre insulates.

Water runs in under the tyre, through the gaps in the asphalt and the siping in the tyre.
So some ice builds up, but then the ice forms a dam, and traps water under the tyre.

The trapped water then freezes and expands 10%. The truck is now on ice…

How to detect this phenomena ? Brush the snow away and look for ice where the tyre was parked at first.

If the tracks left by the tires show the actual tread pattern, then I would think it is rolling. If the track is smooth, then I think it is sliding.

I’ve never seen or heard of a car sliding the way Isilder is speculating. If that slight grade could cause a parked car to spontaneously roll that far due to the freeze/thaw cycle, every new winter would bring thousands of stories of cars sliding into the street and real estate agents for every house in the snow zone would advertise the flatness of its driveway. People would also have invented car tie-downs to keep this from happening, but none of that is necessary.

I suspect a broken parking pawl like beowulf, or a worn shift mechanism that keeps it from completely going into park. Ever throw it in park before it completely stops moving? That could break the pawl. Or it could just be worn out.

Even push-button 4x4’s still go into neutral between 4wd High and 4wd Low. If the transfer case is in neutral, the truck will roll even if the transmission is in park. With the old “stick coming out of the floor” style transfer cases, it used to be a serious problem because you could easily accidentally bump the stick while you were getting out of the truck and cause it to slip into neutral and roll away if you didn’t have the parking brake on.

Is it possible you somehow put it into 4wd Lo and then switched the truck off? (Or alternatively were in 4wd Low, and switched back to 4wd Hi or 2wd then turned the truck off before it finished shifting?) I’d assume the truck is wired such that if you do that it can finish moving the transfer case into gear without the ignition on, but who knows. The push button transfer cases also can just be temperamental and sometimes get temporarily stuck in neutral.

Parking pawl does sound like the most likely issue. I have been taking it in and out of 4 wheel drive a lot lately so it could be a malfunction there.

If it’s the pawl, how hard a fix is that? Rebuild the transmission hard? In all likelihood this truck gets replaced this year so I don’t want to throw too much money at it. Using the parking brake the rest of it’s life seems to be the likely solution unless it manifests into a bigger problem.

Broken parking pawls are not very common and wouldn’t be intermittent, so assuming the truck seems to goes into park normally now, I seriously doubt that’s what it is. Just make sure when you park the truck that the gearshift is firmly locked in park, the parking brake is on, and the transfer case isn’t in the process of shifting when you turn the truck off.

We are taught to ALWAYS set the parking brake.
parking the car during your driving test and failure to set it would mean you failed the test
Every car I’ve owned, manual or automatic, setting the handbrake / emergency brake / parking brake is done everytime