Phil, chock the front tires and jack the rear of the car up and put jackstands under the differential housing tubes so that one of your buddies can slide under the car safely. Have him make a chalk mark on the driveshaft and make a pointer to point to the chalk mark out of a clotheshanger. Put the other end of the clothes hanger under a brick to hold it in position.

Make a mark on one of the tires and do the same clothes hanger trick to mark the spot on the tire. Now, turn the tire slowly for 10 full turns while your friend under the car counts the number of complete turns of the driveshaft. Have him guesstimate the fraction of a turn that he has left when you reach 10 full turns.

Let's say that the driveshaft turned 37 3/4 times. Divide 37.75 by 10 and you get 3.775, which is probably a 3.73:1 rear.