I also told my dealer "no decals", the salesman said no problem he'd make sure of it, but they still put one on along with the license plate frame. I refused to drive it off the lot until the salesman took off the decal, then I took off the plate frame when I got home.
However, to answer the original question... I suggest carefully picking at a corner with your finger nails to get it started, then pull it off. If the decal is actually individual letters do the same thing to each. There will probably be some glue residue left on the truck. I found that lighter fluid takes the glue off very well, and it won't harm your paint job. Just dampen a small rag with the lighter fluid then gently rub off the glue. Hopefully the decal hasn't been on too long, otherwise the paint may have faded around it. Get it off as soon as you can!

Al