I have a 04 F150 fx4 and When i put my truck in 4 wheel drive the front wheels will start to turn but when i get them spinning faster i hear a clunk from underneath and a grinding sound from my transfer case and it kicks into 2wd. I only have the problem when im in drive. In Reverse the 4x4 works perfect. I lifted the wheels off the ground and kicked it into 4wd. They all turn till i grab ahold of one of the front tires then it starts the grinding again. What might be the Problem. HELP PLEASE