What Longshot is pointing you to is the "VIN sticker". You'll see a code under the word "Axle". Mine is B6, which is a 3.73 Limited Slip differential. These are the OEM codes:
19 = 3.55 open
H9 = 3.55 LSD
26 = 3.73 open
B6 = 3.73 LSD
27 = 3.31 open
18 = 3.08 open
25 = 4.10 open
B5 = 4.10 LSD
Aftermarket axles can have different ratios.
Some of us, like me, got "smart" by haunting these forums and then reading everything I could find on the new vehicle designs. When I got my truck, I decided it was running on "magic" since there was no distributor, plug wires or throttle cable!
It's terrible when you find out everything you learned about vehicles back in the 60s is all meaningless!
- Jack