Lawn and tree guns generally are not interchangeable although I have sprayed lawns with fungicide and insecticide with a JD9 and another small T/S gun I have.
You can spray shrubs to some extent with a chemlawn lawn gun although not ideal.
Buy two guns. Chemlawn gun and a JD9 or knockoff. Shouldn't have to spend more than 150 - 200 for both.