One of the simplest solutions is to have the axles at different angles.
This is easily achieved with something like this: http://gallery.scioly.org/details.php?image_id=2923
Take a look at the front end of the vehicle. This allows the angle to be adjusted.
Or you could have the vehicle be built in two separate parts, and join them with a bolt in the middle. The bolt is vertical, and both halves are attached on it. This allows you to set the angle of each half (each one has a axle) and tighten the bolt to lock the angle in place.