I know tjis is a F150 forum but I need some advice badly. My 99 Expedition has all-time four wheel drive. Whenever the back wheel starts to spin, and the front wheels start to grab, you can hear a loud popping toward the front-end. Especially when I accelerate on wet or slick surfaces. It feels like the 4 wheel drive is not kicking in all the way. It sounds like it could be expensive! Please help!