creeping automatic

[3bears]

last night, on an old forest service road, I stopped the truck and put it in neutral and noticed the truck starting moving backwards. At first I thought that maybe I was on a slight down hill , and let it just keep creeping to see. Well it continued to creep and even started creeping up hill backwards until the idle speed would not let it go anymore.

Otherwise my trans has not had any issues since I have owned it. I have flushed it and switched to synthetic a few months ago.

Ideas ?

 

[DK327]

I have flushed it and switched to synthetic a few months ago.

I was under the impression that for 3rd gens with high mileage drain and refill is recommended and not a complete flush? Not saying that's what is causing the issue, but just wanted to throw that out there...

 

[3bears]

It was not a power flush, just a flush like many do on this forum