Alternator is out
. If you wanna check it put a voltmeter to your battery while the engine is running. Should be at 14 volts anything lower is more then likely a bad alternator. I am pretty sure that your local auto zone or murrays will test your old one for free. If it was me I would buy the one with a lifetime warranty if you plan on keeping the truck for a little while.
...................just my .02