1986 Dodge Ram 50 won't start

I have a 1986 Dodge Ram 50 pickup truck, and for the last few weeks it's been driving okay, with the exception of it sort of shaking when I stop at redlights, and last week when I parked at work, after putting it in Park, turning the key and taking it out of the ignition, the truck made a shuttering noise as though it was still trying to go, then it finally turned off. After that it still drove okay, but today I went to warm it up and although it started, when I went back outside after five minutes it was dead. And now it won't start at all. It makes a noise like it's trying to start, but nothing happens.

Anybody have any idea what might be wrong with it? I'm not sure what the engine size of it is, as I don't know much about vehicles. I do know that it's not the original one from 1986 though, because I purchased it in 1999 from someone who had just put in a new engine.