Detonation on 2001 GMC Yukon XL Denali

I seem to be experiencing a lot of detonation under minor accelleration both around town and on the highway. I also have the piston slap at startup, but the noise on the road sounds more like detonation. I've tried high-octane gas, but that hasn't helped. My truck has 190,000 miles on it, and has been well cared for all of those miles. Any ideas on decreasing the detonation?

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