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You need a new gear. If you have a 2wd, then follow the speedometor cable to the trasmission, and if you have a 4wd, follow it to the transfer case.
It shouldn't be off much, Just show your going about 75 when your really going 70(and that should be about the worst).
But either way, broncograveyard.com sells them, and they have a way to calculate which toothed gear you need to swap in place.