you need a diesel engine, and you need to refine the veggie oil to get certain chemicals out of it. it can run on straight veggie oil, but it will clog up certain parts. i'm not aware of the specifics, you can get more info in the "alternative fuels" section of yahoo answers.
and as i said before, it will never work with a gasoline engine, not enough compression. you need a diesel engine.
and yea, i'd personally sell the jeep, and get a GOOD SUV with a diesel engine.