I suppose once you rule out anything electrical then it very well could be a problem in the clutch packs (forward pack)
The only way you'll know for sure is to take pressure readings to confirm if it is a mechanical failure or not.
About a year or so ago i had a cat truck with this exact symptom and in the end it did turn out to be in the clutch packs. A reman tranny and it was back to normal.
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