8 speed was the nicest towing transmission I have ever had in a half ton. Always knew what gear it needed and when you dropped gears going down a grade it stuck like a magnet with basically no input of the brake pedal. I had 3.92s in it.
As for it's absence in the HD line up- it has been covered in depth on a few forums. It has to do with the ability of the transmission not being rated to hold the full GVWR of the truck when parked- so i assume the pawl or something was the weak point?