Lift rear tires off the ground, put truck in neutral. Put a mark on drive shaft and rotate wheel one full turn and count the number of times driveshaft completes rotation. That will give you close approximation of what gear you have without opening rear cover.
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...