Searching the net with google for "vehicle advertising vinyl" I got a few hits, one of which is adsonwheels.com.
They charge to register your vehicle and put you into a pool for advertising, but their terms don't seem to be too bad. I did note that you generally cannot move, have to drive around 800 miles or more per month, and may be required to have a battery powered GPS unit installed depending on the company doing advertising. They send you a 1099 at end of year, but don't deduct taxes from the monthly payments. There are other restrictions....
BUT THEY PAY $450 A MONTH!!
Hmmm... I can handle paying $50 (net) a month for my X car payments... no problem.
The X should be a good truck to advertise on, cause it's got a high profile and lots of space to advertise on.