To my knowledge, the 4.5 was a completely new engine being developed by GM for the 1500
series trucks and 'burbs during the 06-07 time frame. The economic crash wiped it out, at least for next few years.
I ask for a Duramax 2500 at every GM event (mostly factory displays at NASCAR races) and they just look at me and say nope.
I would love one, too.