I reinstalled everything the correct way, pretty sure. I drove it for the first couple weeks after HG replacement without any troubles. I hadn't checked compression at that point, however.

All of a sudden, one morning when I started it up and drove it I could hear some pinging or valve noise when under load.

I just talked to my wife's cousin, who is a Toyota tech, and he suggested that the rings are going for whatever reason. He suggested that if it's using oil, which I think it is but haven't verified for sure yet, then that oil (when mixed with gas in the combustion chamber) will reduce the fuel octane thereby causing that pinging that I'm hearing. He suggested added some octane booster to see if the pinging goes away. Then the question would be why in the world are the rings all going bad at once? The bottom end is completely stock and has 180,000 or so on it.