Why is my truck running so rough?

My 2002 chevy van has been running horrible, it wont accelerate, it backfires, and now it wont start. We have changed the fuel pump, rotary cap, spark plugs, wires. the fuel filter, air filter are both fine. I have no idea what else to check, any ideas?

You need a good independent mechanic. The ECM (computer) may be faulty or the catalytic converter is plugged up are two areas that come to mind.