Can you get the code for the axel? I would go with either 3.08 or the 3.73. If you can't get the code, raise the entire rearend, put the truck in neutral and turn both wheels at the same time. Count how many times the drive shaft turns in relation to one turn of the wheel. You can get an approximate estimate. Since you just bought the truck, I would change the oil in the rear end. Remove the cover and count the teeth or look for the numbers stamped on the ring. Devide the smaller into the larger number to get your ratio.
I have a 12 bolt that replaced a 10 bolt. It also had heavy duty springs and it's a 3.08.
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Gerardo
1983 Custom Truck
TX Plates: "BAD BOW"
http://www.geocities.com/abetterchemist/