It could be an internal issue. However, if you can shift it manually, the clutches sound fine. The first thing I would check is the transmission fluid. Check to make sure it is full and its condition.
Also, I am not showing this vehicle as a "truck". What model is it? Was the transmission fluid ever changed?
Let me know.
Take care and God Bless,
Joe
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Monday, March 15th, 2021 AT 6:20 PM