The chalk mark on the driveshaft method works, but its sometimes off. Sometimes doing it that way is hard to tell the difference between 3.73's and 4.10's, or 3.42's and 3.73's. The best way to make sure you know exactly what the gear ratio is, is to pull the diff cover. There will be a stamp on the outside edge of the ring gear. There will be a string of numbers. The first one is a long one which is the serial number. Then there will be 4 more. 2 of those are the gear ratio, you divide the two to find the gear ratio. Im going from memory so it might not look exactly like this but it should be something close: (17854623-serial number, then 7 81-this would be the build date july 1981 in this case, then 9 41-this would be 4.56's in this case). It might not look exactly like that (17854623 7 81 9 41), but you get the idea.
-Harrison