I have a 2010 gmc 2500 diesel and my transmission control module went out causing it to get stuck in 3rd gear and shift hard from reverse to drive. I have friends that had different symptoms when theirs went out. Mine went out suddenly driving to work. Even if you have a gas truck you will have some sort of computer control for the transmission. Also check the altenator. High voltage can make electronics do strange things just like low voltage can. It's better to try and think of the cheaper and easier fixes first.
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