I have a 1999 F350 super duty 4x4 dually. Twice I have put the transfer case in the low range position to pull a trailer up an incline. When I finished the pulling I put the transfer case back into the high position but it failed to go into high range. I had to take it to the Ford dealership to get it back in high range..The dealer said it needed a new switching motor and a new gem module. I replaced the switching motor. About 4 months later I again put it in low range only to have the same problem with it not coming out of low range. Is the problem the gem module? Does it control the switching of the transmission or transfer case?