How di I decipher the numbers on the rearend tag? How do I tell what gear ratio it is? The tag reads (top line) WDM-CJ-0C4. The second line reads: 75 9 s314b. What does this mean?

 

Is it the original gearing that came new from the factory? If so, the info will be on the tag in the driver's door jamb. Look for where it says 'axle' and there are several websites that list the codes. I don't have any of them offhand but I'll find a web link or two and post them here this evening (if someone else doesn't beat me to it).

Here, see if this helps:
http://www.ford-trucks.com/article/i...r_Decoder.html