I have had the most luck by sanding them off with a block and 2000 grit or so paper. But you REALLY have to be careful to sand ONLY the letters and not the surrounding paint.
But first, see if you can wipe it off. If the truck is an OEM paint you could take a lacquer thinner soaked rag to the letters without damaging the paint under it. Give that a try first. Just test out the paint with a thinner soaked rag under the bottom of the door or something you are working on to be sure the paint IS OEM and you shouldn't have a problem.