What is this thing?

I just bought a 1987 GMC Sierra3500 and this thing was hanging down from under the dash by the brake.
anyone know what it is for?

neutral safety switch. It's for automatic transmissions. So the starter will only crank in park or neutral. Is your truck an automatic? It should bolt up the the steering column. If you have a stick then someone might have changed the steering column and you don't need it. If you have an automatic, someone may have changed it and didn't get rid of the old part.

does the truck start normally?

yes it starts fine,idles ruff,seems like its running to rich but can't figure why

thanks for the info