It's the RAVELCO anti-theft device.
http://www.ravelco.com/
I HIGHLY recommend it to any one who has alot of monetary or sentimental value in their vehicle and if they are in an area with any level of crime.
About 2 years ago to this day my dads 6.0l was stolen right out of the parking lot of his work. The truck was gone and all he was left with was a $22k check for truck that was only 2 years old. As you know N.A.D.A or KBB values would make it hard to get anything more than $11k for our trucks.
So now we have all our trucks outfitted with them. Never has a vehicle been stolen with this device. The grey cap has a series of pins in it that are specific to my truck and there are about 100,000 different combinations. The inventor of the device actually lives a couple miles for me. He installed it in my truck himself.
I'd definitely check it out if you fear having your truck stolen.