The service light came on a week after I'd purchased my truck, used. This was about a year ago. I tried engaging the 4wd and nothing happened. Switched back to 2wd drive. Driving down the freeway (about 70mph) the 4wd tried to engage and destroyed the transmission. Transmission replaced. The service 4wd light still comes on and I'm hesitant to try and put it in 4wd again. Is the FCM bad?