From an engineering standpoint it seems like it is just a weight and packaging problem to me.

The Pacifica PHEV weighs about 5,000lbs depending on configuration. I think the only deficit it really has to the Model X is drag co-efficient but Chrysler was working with a given shape. Given a unique platform not shared with a gas variant they could design for a specific co-efficient as needed.

I don't really know the specs of the Model X, however Google says it weighs maybe 300lbs more than the Pacifica. Those aren't unsurmountable differences - especially if the price range is going to be 80k+.

I think the biggest difference is that Tesla knows it won't sell enough vans to make it worth their development time :)