[JackandJanet]

Here's a list of OEM rear axle codes. You can find the code for your truck on your driver's door sticker.

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

- Jack