Ya, you should not need premuim fuel. The 4.0 is designed to run on 87 or 89 octane. Using higher octane might mask the symptoms, but the problem will still exist.
You might have a bad sensor (O2, TPS, etc) somewhere sending erronous signals and causing the computer to set the wrong timing curve.
How many miles have you got on it? You might also just have some carbon buildup causing the detonation.
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