The reason I know it's doing 76 when it says 70 is because my mom was driving beside me in the '02 Yukon, which I highly doubt is off at all. But I could be wrong. The tires are the same size as stock, cept for they're 235's instead of 215's, which I understood wouldn't affect the speedo.
Now that I think of it, I have had the diffy replaced. The old one was grinding/rumbling weirdly from 70 on up, so we got a used one. Any more ideas or fixes?