Cbus, I agree with your analysis...I will give it one more try on Shell 87.
In Southeast Virginia, probably like the rest of the country, we've gone from it being very warm to very cold quickly, several times...which, of course, causes a lot more condensation...so there is that possibility that my tank might have more water in it than usual...and therefore the gas is no longer really 87 octane -- that is it's been slightly diluted...but if a new tank of it doesn't solve the problem, then I will switch over to mid-grade (89 octane). Again, the pinging only occurs at when accelerating at highway speeds and partial throttle (i.e. giving the gas to jump from 55MPH up to 70-75MPH)...I haven't really heard it on wide open throttle (of course the exhaust sound could be covering it up).