ECM Failure ?

99 Limited, 266k, Factory alarm? No CEL, No remote starter.
Back history, Randomly the alarm would start, flashing headlights n Horne until the battery died. Once the battery was charged the truck would attempt to start with no key in the ignition. Unk if this is fixed. Changed out the starter relay, no luck

Today’s new problem
In Park, using the key
The motor will turnover, start, the starter stays engaged

Shift the transmission into Neutral, the starter engages.

Shift the transmission into reverse or Drive, the starter disengages

If I switch the truck back to into neutral, the engine shuts off.

Once the The truck has been running for 15 minutes or more in drive, you can put it back in park or neutral and the starter will not engage. However the truck will shut off immediately.

 

If, when
you start the car, the starter keeps trying to crank the engine ...I'm thinking your ignition switch in the column is shot.

At that point I'm pretty sure the ecm doesn't control the starter.

 

So rereading and trying to make sense of this issue, this line struck me as odd. What do you mean by this exactly? You drive it for 15 minutes. Put it in Park. Then try to start the car while it is running? With the key? When you do, the car shuts off?

If it's not something like a bad ignition switch doing all this, I'm going to bet that someone has royally f***ed with the wiring on this car. I bet when you get in the dash the story will write itself.

Either that or find a priest. You may need an exorcism. Posessed by demons or something.