My 4x4 has failed it just flashes and it reverts back to 2wd

As a fellow technician, you know the importance of proper diagnosis and testing. The proper way to begin here is with a scan tool to check the transfer case module. Only way to go with this one! We could start guessing at your problem based on similiar repairs that have been performed previously but each is unique and that doesn't really help. Have it tested with a professional grade scanner, not a code reader.

My truck did the same thing; it wouldn't shift into 4WD. I took it to an independent transmission shop locally and he fixed the problem by replacing the actuator with an aftermarket heated actuator. He sais the fluid would congeal and gum up the actuator (this happened in the winter in the northeast US).