If I wanted a vehicle for off-roading, I'd either see about a special-order Roadtrek 190 Versatile which has a 4x4 option from Quigley, a Sportsmobile model, or would go with a 4x4 pickup truck and TC as another option.
As of now, the only Euro-van is the Sprinter on our shores, and M-B will pull any authorized dealer agreements if someone tries to sell a 4x4 retrofit, otherwise Quigley and SMB would have already have had upfits for the model.
The ProMaster is FWD, and very difficult to convert, so most likely the vehicle of choice for 4x4 upfitting will be the Transit.
I'm sure that a Transit upfitted by Sportsmobile will have enough clearance to handle a Powertech disel generator (if the engine is a diesel), or mount a gas Onan (if the van is an EB V6 or the Flex-Fuel engine.)
Of course, there is a rumor that the 2015 Sprinters shipping to the US (likely next September, 13 months from now) will have 4WD/AWD as an option, but as far as I've read, that is just one source.