Hello Kory,
We use a D-Limonene based product. AR 409 Orange Peel (Adhesive Remover).
Check with a sign company in your area. The product is kinda pricey ($40/gal or so), but it does the job. We have removed lettering from several hundred fleet trucks and cars and in most cases you can't even tell that it once had lettering at all!
Dave Olson