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We can't help you with what little information you've provided. Do some things like:
*Obtain DTCs from your truck if present
*Check the transmission fluid. If brown or dark, it's burnt and probably needs to be changed, along with the filter.
If there are no codes and the fluid is ok, I would imagine it needs to be dismantled and properly diagnosed. Not something a home mechanic is capable of, take it to a shop that specializes in transmissions.