If an octane change made a difference, it sounds like pinging.
Have the dealer reflash the computer to retard the timing. Sk the
dealer if a TSB has been issued for this problem. You might lose a bit
of performance, but at least you won't need premium fuel.
Replace the thermostat with a cooler one. Some people have even had
success with re-routing the spark plug wires.

John

   If 92 octane helps, then it's not valve chatter - that's pre-ignition.  Valve
chatter sounds kind of like a sewing machine.  It's a pretty steady sound, that
varies directly with rpm's.  It's caused by slack in the valve train.
    Pre-ignition - knocking - may be present all the time but rarely is.  It is
worse under a load, regardless of rpm's.   If it's there all or most of the time,
check for a vacume leak - especially in the belly pan.

you may want to check and see if the plennum pan gasket is leaking. there
was several tsb's on these trucks for simmular complaints such as rerouting
the spark plug wires. but the one i personally had best sucess with was the
plennum pan. simple to check just pull the pcv valve out of the valve cover
put your finger/thumb over the opening in the valve cover, if you feel
vacumm in the crankcase then the plennum pan is suspect. at that point its
just a matter of pulling the intake and replacing the pan gasket on the
bottom of the intake.

also you may or may not want to consider a combustion cyl cleaner. mopar has
one and if you google this news group you can find posts discussing
procedure on using and the successes of its use.