The diffs had (operative word), and with any luck will still have metal tags retained by one of the inspection cover bolts. These tags would indicate what the gear ratio is. That's if they haven't broken off or been removed. For a good guess, you can get both rear wheels up in the air (safety first), make yourself some reference marks on the rear drive line and one of the tires. Take the trans out of gear and rotate a wheel one full turn while counting how many rotations the drive line makes. Three and a half shaft turns to one wheel turn would be 3.54. Three and three quarter turns would be 3.73. Slightly more than four turns would be 4.10. Only way to be certain is to pop the diff cover off and read the numbers stamped into the ring gear.