Savage x 4.6 motor runaway

[Kryptek1]

I was using my savage and next thing you know the motor went wide open but the truck didn't move at all I had no control over it had to plug the exhaust to get it shut down what would cause the motor to do that I had aluminum clutch shoes the slipper was all the way tight any help on this would be great

 

[J.LO's SAVAGE X]

I would check the nut that bolts the fly wheel down, sounds like it may be loose, happens often, may have to take clutches off