1. "diesel" engine ran on peanut oil, and the first "diesel" cars were ran on vegetable oil. so...any diesel will "eat" vegetable oil, there's no big conversion needed, the only difference is, vegetable oils burn hummm, cleaner and they will eat up natural gum, so you have to replace those tiny hoses with silicone ones, i think that's basicly all the conversion a car needs, also, the tank might be painted on the inside so it might be wise to at least check if it is.
reinbeau, the part about taxes, they have been payed, although not for the energy, but they take good care about vegetable oils being more expensive than diesel, or at least at the same price. i don't know how you have it in the US, but biodiesel is available around here, so they can't hold you on this, but the oil used for heating is the same as diesel, only that it's colored and it is at much lower price, so if you tank the heating oil...and get caught, you pay, a lot.
asprince, you probably know you're not doing any favour to your car do you? i mean, just filtering it isn't enough. although if the truck is a bit older and you're not "saving" it, hell why not, why complicate things.