3 things will cause your issue. Wrong tire size which you have confirmed are correct. Wrong rear gear, not sure if you have confirmed that. Or the ecm does not have the correct ratio programmed into it. I have a 97 K1500 that is lifted with 35 inch tires. It had factory 3.73 gears that I changed to 4.10. I bought a Hypertech programmer that I could change tire and gear changes. Not sure if you can find a used one, but it might correct your issue.
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