it sounds like you don't have a Saf-t-track rear. jack up one side and count one revolution of the wheel. with that rotation the drive shaft will turn half of the total revolutions. Say you have a 4.11 gear, the drive shaft will spin roughly a smidge over 2 turns. 2.41 will be close to 1.2. I divide the drive shaft in 4 quarters and treat it like a clock. quarter hour, half hour, ect. in any rate you will be close to what the ratio is.
On a Saf-T-Track, you jack up both wheels and spin it. One Rev of the tire and the drive shaft should turn the full revolutions. 3.73 would turn 3 and 3/4's the way around. Good luck
The codes are good too but you have to look for them on the axle tubes. Sometimes that harder because they don't stamp it deep enough or you have to clean the axle from the grime.