If you have the tag on the front axle, it will be the same (or very close) to the rear ratio. You should have a Ford 9" rear axle, and that normally came with a 3.50:1. It's remotely possible that it's something else, like 3.08, 3.73 or even 4.10.
If you don't have either tag, then you will have to jack the rear wheels off the ground. Count the number of turns the driveshaft makes for one revolution of the tire. Turn the tire by hand, and leave the motor off. Might need to put the transmission shifter in neutral, too.
Mark a spot on the tire, and go around twice. Mark a spot on the driveshaft, too. The 3.50 ratio will mean that the driveshaft turns seven full times for two full rotations of the tire. If the opposite side wheel turns in the other direction of the one you are turning by hand, then you also have an open differential, or no limted slip or locker.