Based on the best of my knowledge, I think both of your choices are on the Sprinter. From dim memory a few years old, I think I recall that the 4x4 Sprinter conversions didn't work because the traction control & anti-lock (& maybe stability control) computers won't play nice with the 4WD hardware. Sportsmobile may or may not have a successful version.
B's based on Fords and Chevys can easily be converted if you have the $$$, tho' it's easiest done on a new van. Sportsmobile makes 4x4 Fords, and Quigley used to do Roadtrek Chevys for RT; they may still do one-offs to order (?). I think I'd expect about a $15K upcharge.
The next generation of Sprinter is supposed to come to the US with optional 4WD. I don't know how far in the future that will be, and I'd expect it'll be with a hefty price premium. How soon B's hit the market on that platform I have no guess. I'd caution that rumors of 4x4 Sprinters have seemed to be mostly made of vaporware.
You should look at the Tiger Bengal CX & TX. I bought mine after 2 RT's, and still love it after 8+ years.
On edit: Try this link from the C forum. IIRC from reading it a while back, there's no real mention of which chassis weight rating they're talking about.
http://www.rv.net/forum/index.cfm/fuseaction/thread/tid/27619768.cfmJim, "My typos, mistakes, and misspellings are intentional and copyrighted."
