This is a viable idea although I'm pretty sure nobody has gone full-scale commercial with it.
BTW sorry to be late to the party but just joined. Here's a page that I googled up because I had heard of systems like this before:
http://www.caranddriver.com/reviews/...hybrid_feature
It's called a "through-the-road" hybrid, and the article claims it's patented by DaimlerChrysler, though given the change in that company since the article was written in 2000 I'm not sure who owns that patent now. Not that a patent will stop an individual from knocking the idea off on their own vehicle, but somebody holding out for a company to make a retrofit kit might have a *very* long wait.
The Durango used an 89 hp motor on the front wheels and the ICE through the rear wheels taking advantage of the standard 4x4 architecture.