There should be a tag on one of the bolts holding the cover on the differential case. A dealer can take those numbers and tell you what you should have (assuming it was never replaced).
Another method just as easy is to do this:
jack up the rear end so that both rear wheels are off the ground and supported safely by jackstands. Put the transmission in neutral with the parking brake off (front wheels chocked unless it is dead flat ground).
Make an index mark on one rear wheel using white chalk, marking the wheel near the frame or something where you can accurately tell when it is back to that position. Mark the driveshaft in a similar way at the rear U-joint. Now for the fun part.
By hand, turn the drive shaft carefully counting the number of revolutions until the wheel has turned exactly one turn. It will likely take 3.something turns. Getting that "something" accurate is important if you want an accurate ratio number. To compute your rear end ratio, you just need to know how many turns of the driveshaft were needed. three and 1/4 turns is a 3.25 ratio.
For more accuracy, you can do this same thing, but turn the rear wheels 10 complete revolutions exactly (good wrist / forearm workout turning the driveshaft). Again count the number of driveshaft revolutions, but now you don't have to be as accurate on that last fraction of a revolution. Suppose it almost exactly 35 revolutions. Divide that number by 10 and again that is your ratio (3.5, or more likely something pretty close to that.)
For perfect accuracy, you remove the cover, remove the carrier/ring-gear, and count the pinion teeth and divide that number into the number of ring-gear teeth. But that's both messy and takes a lot of time. Turning the tires 10 revolutions will get you a very accurate answer.
Thanks for all the info!!!! I checked the Vin...Turns out I have the 3.27s (LE code). That's good news...I was afraid I might have a worse gearing My Stang appears to have most options...17 inch rims, ABS, leather...etc.
I bought it only a few days ago...took me two days to polish and wax it...it's all shiny now
Time for a few mods