If you select the truck, you can go to Select -> Modify -> Contract. then just set your contract to about 2 px. From there you will want to inverse your selection (so that you don't have the truck selected, but everything around it). You may now wish to feather your selection just a couple of pixels. Once you've done that hit delete. I think that is the method you were describing. Sometimes it works well...sometimes it doesn't.
Originally posted by MCD4x4
Someone mentioned to me that there's a way to inverse somehow and take off 2px to clean up the halo. I just don't remember what they said.
Edit: Also, just so you know, this method doesn't work too well when you are using a transparent gif. The anti-aliasing is lost when you make a gif transparent (as bradyj explained) and thus you will get a halo.