I was working on a DT like this a few years ago (right after the 06 season, I believe). Only mine had 8 wheels, and both ends folded up. They were designed for one specific angle, and pneumatically controlled. If they are used elsewhere on the robot (or if you just have the weight), I'd strongly recommend using pneumatics to lift the end(s) of the robot. The complexity of the system is greatly reduced (ya know, less parts there are, the less parts there are that can break), and there's less stress in the system (you're currently overcoming gravity AND the gas shocks). Plus, the time it takes to prepare for ramp climbing is greatly reduced.
If you don't have the extra weight for the compressor, accumulators, etc, then I'd recommend either a belt driven system, or a spool with some sort of cable. Any motor could be made to do the job, it's just a question of how fast or slow, and how much weight for gearboxes you have. The amount of thought required for the motor option is several orders of magnitude higher than for the pneumatic options.
As for the hinges.. Bushings should be fine. The shaft in the bushings won't be rotating too quickly, so they should have a sufficient lifetime. Also, there shouldn't be much of a radial load on them, so they shouldn't seize up while in use. Simple, but effective, IMO.
I've supplied a link to a few images of the DT I designed a while ago. It's a smaller scale, and the outer wheels aren't driven (though the large scale one I was working did have 8 driven wheels) . I didn't put in the inner four wheels, but you can kinda guess where they would go. Unfortunately, none of the pneumatics are there or anything. But it looks cool.
Folding Drive Train