I think you should strongly consider the
AndyMark 446 Drive Module. You get a COTS two speed drive module with gear drive for the four center wheels. You shouldn't be too scared of the chains that drive the outer wheels, because you're still mobile even if all the chains break, similar to a West Coast Drive.
If you want two speeds and a long robot that can turn, completely eliminating chains/belts will end up adding a lot more complexity than it removes.
If you want zero chains, simplicity, the ability to turn, and traction, I only know of one way to do it -
these nanotubes (also mentioned above). Those are single speed, and you don't get the power from all four drive motors unless the correct four wheels are all touching the carpet.
Or just do kitbot on steroids. Or find a custom way to direct drive the center wheels of a 6WD to eliminate two of the chains. Like Eric said.