When you're talking about a 10 year old vehicle you're always going to have paint fade. The paint under the decals is going to be darker than the paint that is showing and buffing won't help to fade or blend the two shades.. Even under perfect climate controled conditions with UV from the artificial lights you will still get enough fade in the exposed paint to show where the decals were.
The only two options you really have if you remove the old decals is to either A) Remove the decals and get the truck repainted or B) replace the decals with a factory sized decal kit that can be laid right over the place where the original decals were.