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Electrical will not engage anything to keep the transmission from free spinning. The only thing that it can do is lock in and not unlock, but that is usually the pump or the relief valve on the shift plate, but I don't suggest anyone that is not familiar with the transmission to remove that plate unless you have the tools and the knowledge. That thing is a pain to get lined back up right.
My suggestion os to drop the drive line and tow the vehicle. Now if the rear is locked up and not the transmission, that is a whole other issue. Then the rear needs to be pulled apart because most likely one of the gears has broken and bound up. That happens quite a bit in these medium duty trucks from overloading.
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