Check your EGR for proper functioning. The passage from the EGR valve to the EGR hole in the floor of the intake plenum, behind the throttle body, must be clear. Mine was all carboned up. Lack of EGR can cause high combustion chamber temps, hence the pinging you were getting.

Check the harmonic balancer, if the pulley slipped you could have been unknowingly giving it too much timing advance.

Check the cat and muffler for clogging/restrictions.

You might also give Jerry Lemond a holler, I bet he's seen this type of problem before.

Good Luck, let us know what the autopsy reveals.....ed