[GarrettF2395]

My team (as of the last meeting) is considering a drivetrain similar to this...

___.........___
|==\....../==|
|....|.....|....|
|[]...........[]|
|[]...........[]|
|................|
|................|
|==.........==|

The == are non powered horizontal wheels that don't normally touch the ground.
They are only there to keep the chassis from hitting the ground during impacts and abrupt stops.
The [] are powered wheels.
This is essentially just a 4WD, but the wheels are as close together as possible for better turning.
They are slightly towards the front because that is where the majority of our weight is expected to be. That way we keep as much weight directly over the wheels as possible. It also helps balance things, so it has as little rock back as possible.
This (or something like it) may be a good fit for what your team is trying to do as well.