I know you're thoughts on GM but if you want an excellent diesel engine Duramax/Allison is a great combination.
I have a 2007 1 ton CC Dually and love it. I had no problems and average 10-12 MPG pulling. Remember,Duramax is Isusu.You might want to read this.
http://www.isuzu.co.jp/world/corpora...e/history.html
Jack