Check out this link:
http://bankspower.com/products/group/14-SpeedBrake
Gale Banks engineering finally came out with a brake system for some newer model Chevys and Fords. Reviews are also pouring in and what a system it is. It is so customizable that you can actually go downhill, that is steep and long, with your load, never tap the brake and actually slow your rig to 25MPH. Now THAT'S braking. Or just preset a speed you want to maintain on the way down and it's all taken care of for you (What happens when you get close to another rig going slower? More to read).
I looked up my truck and the system is under $1,500, ouch, and I don't think it includes installation! This is the bad news. Similar systems are about $800 to $900 from BD Diesel or PacBrake for my truck. But the Banks Brake comes with a computer, big screen and other customizable stuff, which I have to figure out if I really need it that fancy.