You need vibration dampers or isolators. I'm guessing they would be fairly expensive if you could find something that would work "off the shelf".
You might be able to DIY using cheap vehicle shock absorbers. Maybe. You would have to try it. I would try pneumatic shocks. Attach four (or at least three) to two plates, one plate attached to the truck, and the other to the jib. That might actually work well. And you might be able to DIY fairly cheaply and easily. So if it doesn't work, or work well enough, you aren't out a lot of money or time. (relatively speaking) That's just quickly off the top of my head.
Passive isolation would be cheapest, and maybe easiest, I think, and any active isolation (feedback with automated electro-mechanical compensation) would be more technical and difficult and probably (much) more expensive.
There is also the question of "simple vibration" versus "shock and bounce". (a relatively smooth road versus potholes, etc.)
Depending on how heavy the jib and camera setup are, you might be able to make your own fluid damper, by placing something inside of something else filled with water, and tied down so the two containers don't touch. (heavy duty rubber bungees? maybe with soft foam fillers (wet or dry) to help with lateral movement)
For example, if it was a tripod with a camera on it, you could use three cans that the tripod legs go in, with those in three larger cans filled with water, and the tripod "tied down" to the truck bed and sides with bungees so it can't really move, and is forced to float on the "bed of water". I would think that would work really well and pretty much remove all vibration, and small bounces.
Here's a link to a nice, expensive book that might be overkill for you, and a few other links that may or may not be of interest or value and/or give you some ideas, and then Google some of these terms I mentioned. Good luck.
http://catalog.asme.org/books/PrintBook/Passive_Vibration_Isolation.cfm
http://www.shopatron.com/product/part_number=7368/1323.0.42062.0.0.0.0
http://www.gelmec.co.uk/GelmecSiliconeGelStudMounts.htm
http://books.google.com/books?id=LoyyOWJzZiYC&dq=vibration+damping+or+isola...
