Ford Repair: knock and ping, engine displacement, egr system

Question
I have a 2004 Ford e-350 van cutaway. In past several weeks when I drive up hill or or with load the engine knocks real bad, when I take foot off gas a little or it shifts it dies down, not completely but almost gone. Engine does not tap when idling, oil level is fine. Could you lead me in the right direction? Also if I do not have this fixed right away will this cause damage? I will bring the truck into Ford if this is something that I cannot fix myself.
Thanks

Answer
First off, try switching to a higher octane fuel. I know my Towncar hates 87. That should cure you up, with any luck. The next thing you wanna have looked at if the problem doesn't go away is the ignition timing. Over time, the timing chain wears, and stretches. Is the check engine light on? It may indicate a problem in the EGR system. Also, if you don't care, could you cite the engine displacement?