Chinook offered 4x4 in our model year (Chinook Baja). The prior owner of mine installed a 4" lift because he took it and ATVs to Moab. I've been happy with the extra ride height although I also added a rear Hellwig anti away bar. (Many folks do this regardless of rig height). I also have rear air bags that the prior owner installed to replace the mor-ryde suspension that came stock from Chinook. I keep 40-50 PSI in it. Higher amounts result in more "bounce" to the ride.
Be careful if you lift it. Action van for example has the proper conversion that assures correct wheel alignment.
. Is the brand that he installed.
You can also have it converted to true 4x4 from companies like Quadvan that use genuine Ford 4x4 parts and thus allow the warranty to not be voided. http://www.quadvan.com
Quadvan has experience with converting RVs.
Quigley is also a very common company that does the conversion, including directly for Ford. http://www.quigley4x4.com