Pinging can occur when warmed up under high-gear (lower-RPM) acceleration if you don't have the EGR installed and everything configured properly. You can try retarding the ignition timing but you should make sure the harmonic balancer hasn't slipped and is feeding you false readings.

Aisde from that, you can re-curve your distributor to give a timing advance that doesn't result in that problem.

I'm assuming this truck doesn't have California emissions? If it does, it's computer-controlled (or was from the factory).