you could use rays as "wheels" to get the distance to the ground at each corner of your vehicle. If the distance is below a certain distance you just apply a upward force to make the vehicle stay airborn

In my car simulation, I have a ray sticking out of each corner which basically acts as a spring. You could definately use such a method, but if the vehicle is meant to hover you'll want these 'springs' to be pretty squashy and long, so when you do a jump you don't suddenly bounce of the air when landing. This technique is demonstrated in my project, which you can see in action here. I'd also add another spring or two under the middle of the body. You might want some aerodynamic force too to push the vehicle down agaisnt the track - and for steering because I wouldn't recommend simulating a tyre gripping the ground for that at all!

I don't have time to explain this properly right know (at work) but if you PM or preferably email the address in my profile I'd be happy to explain better. I've not used ODE but any physics engine will allow you to do this - you're just adding forces to a body each update based on its position wrt the ground.

It's likely to stay bouncy as well. A good approach is to think in terms of a cushion of air under the vehicle - above a certain height it has no effect atall, below that height a lifting force is applied.

More importantly, below the critical height a new velocity-dependant resistance appears for vertical movement (this isn't a force which pushes you up - it only exists as drag when you are going down). That last bit allows you to use quite gentle forces without smacking into the ground whenever you do a jump.