My 2003 Chevy Silverado 1500 is running lean. Before this Happened my truck would just off while driving it. Is my Fuel pump?

You should have 45 psi on the fuel rail. If it is low, it will run lean. It could be a clogged filter or a worn pump.

If your truck is running lean theres a few things you need to check a list from easiest to hardest -Check MAF to see if it is dirty or if theres any pieces bent -Run a smoke test see if you see any smoke coming out of any vacuum hoses or any parts -Also hook up an air compressor to one of the vacuum lines and see if you hear any hissing -While the truck is running pour water around the intake manifold and around your headers or spray brake cleaner either one is fine....if you hear the idle change or if the truck starts idling funny etc then you need to change the intake manifold gaskets or your headers have a leak usually youll see the crack in the headers -also check for dirt in your throttle body or carbon and check the throttle body gasket If your fuel pump is running at 43-45PSI and 50psi while driving your fuel pump is fine and you should also check your injectors there is a stochiometric reading 14:7:1 which is your air to fuel measure your PCM is telling you that your getting too much air and not enough fuel