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do you have a steel driveshaft or an aluminum? Whats your 2K cruise speed? 68-70 or 75+?
When i had 2.73s my 2k rpm cruise was only 73mph on a 245/50/16 tire.
If he has aftermarket rims/tires his speedo wont be correct. If all is stock, then you can tell that way. If not, jack the rear up and rotate the tire and count the revs on the driveshaft.
Start with RPO codes, then if that fails, or been modifed. The 100% sure way to find out, is pull the rear cover and look at the stamping on the pinon should be something like