when i put gas in the truck, the gas pump stops pumping.could it be a problem with the filler neck?

It won't have anything to do with venting, or the fuel pump. The pump handle only clicks off if the nozzle gets splashed with liquid... do you have the nozzle all the way in the filler neck? have you tried other stations? Either something is causing gas to get splashed onto the nozzle i.e not completely inserted, wrong angle, damaged filler neck.... or the nozzle/handle on the gas pump is damaged or malfunctioning. I haven't had a problem on any of my Dakota's but on some models the filler neck is simply shaped oddly and causes this problem.... you can also try pulling the nozzle out of the filler neck slightly, just watch carefully so you don't overflow and spill it all over.

if its not properly vented it will shut off