How to troubleshoot fuel delivery failure.?

When I start my truck up it runs for a couple of seconds just fine and then it just stalls out. I cleaned the plugs (all have spark) and put in a new distributor cap and rotor in. I can keep the engine running using starter fluid, during which the injectors are just dripping but they spray fine right after start up before it stalls. I think It might be the fuel pump or the relay for the pump. I can hear the pump turn on when I turn the key to on but it sounds a little dull (and im guessing thats why it has just enough to start the engine). Im hoping its just a relay problem and not a 100 dollars fuel pump problem with tons of labor.....