FX4 shifting into 4x4 @ 60 mph>

[Fordtruckfx412]

Hey I got a 08 F-150 FX4 and the last few times I drove it the truck will make a weird grinding noise in the front axle randomly and the noise won't go away untill i slow down to 5 mph and ill hear a loud clonk like the truck just shifted into 4 wheel drive but Ill have it in 2 high and after all this its still in 2 high, and I am still able to switch it into 2 high, 4 high, and 4 low. ive never had a truck do this. Has anyone had this kind of issue before?