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It is very unusual for this transmission to lose all engagements in any gear range. The only things that would cause that would be a complete failure of the pump/torque converter or a major mechanical failure of an internal component such as an input or output shaft, neither that I have seen in all my years rebuilding these transmissions. I would be more inclined to believe the problem is in the transfer case, especially if the 4X4 low light is on and you have experienced issues with the light flashing before. It sounds like the transfer case might be hung up between ranges, not allowing transmission torque to be transmitted through the transfer case to the driveshafts. This would be easy enough to verify if you had a scan tool that could monitor data stream pids for the transmission. With the scanner the transmission could be monitored for turbine and output shaft rotation which would indicate if the problem is in the transmission or the transfer case is at fault. The 4X4 module could also be scanned at the same time for 4X4 errors. There is not anything you can do to properly diagnose the problem without a scan tool. You could try rocking the vehicle in park with the emergency brake off - this could jar the shift forks in the transfer case if they are sticking or if the shift motor is sticking. You could also try disconnecting the battery for 30 minutes to clear the modules memory (reset) and then try shift the vehicle to 4X4 low and back to see if that changes anything. Besides that a scanner will be needed to retrieve the data necessary to diagnose the problem.