Take a look at this thread, maybe it can help. RCP57 used a bogie axle setup for the conversion of a bruder dumptruck. The motors may interfere too much with the ground clearance, but having a gear motor for each wheel looks to be the simplest way to do what you need.
http://www.rctruckandconstruction.co...ead.php?t=4870
Maybe you can just drive two wheels on each bogie and use a track over the tires like you sometimes see. Although the tracks will introduce thier own set of problems (see link).
http://www.mascus.com/forestry/used-.../gn3kihfi.html
Either way, you have a great start, I cant wait to see how the rest of it turns out.