I had a similar experience a couple of years ago with "Big Red". My speedometer began reading low - intermittently, at first; then constantly. Then I noticed my fuel economy was really bad - 14's and 15's when I was used to 17's and 18's. And the odometer was reading less than actual mileage according to mile markers. Turned out the speedometer drive gear on the transfer case output shaft was worn to the point that it was skipping one tooth out of seven on each revolution. I replaced the gear, and everything was back to normal.
