I can offer some insights...and I will! Having had two C's...one with one without, for getting up those nasty loose crudy tracks, a 4x4 does that. With only rear drivers, the rears will spin out and there you are. Even with limited slip differential, once you loose the rears, you are kinda done. The ablility of the 4x4 then to drag your sorry a** up and over, make it.
A person can buy a 4x4 kit from UjointOffroad
http://www.ujointoffroad.com/that can be installed. Know a guy that did that and he is some happy. You can always get it lifted..although you`ll need to find a 4x4 shop to do that. There are no kits for a Ford E 350.
There is a good sounding shop in Utah too..or, because you are Washington, take it up to Kamloops Light truck..they did mine. With the CDN loonie so low, it won`t be such a big bite.
Those that recommend their particular brand of 4x4 RV....all great names as long as you have deep pockets. I always suggest to folks...if you don`t have those particular resources or perhaps want a bit more room...buy the C of your choice and have it converted. The bottom line will be a whole lot less money for a whole lot more truck.
And pick an RV that has the plumbing up inside..not dangling down so you can grind it off on a passing rock. It`s also hard to deal with the angle of departure. Mine is all plated underneath, so I can drag it over crud and scratch the plate, not mess up the tanks and so on.
Gary Haupt