How to determine rear end gear ratios

[Robbie]

A few months ago I bought a 1987 Chevy one-ton fleet model R30 truck that had a dump bed installed.  After doing about six weeks of work on it so it would pass Illinois state inspection, I was hauling my fourth load when the right rear axle failed.   I believe it was due to the C clip behind the outer bearing on the brake hub.  Due to the damage that was done, a mechanic friend of mine suggested I replace the rear end rather than try to repair it again.  So, I have put an ad to buy a rear axle assembly on Craig's list.  I have had one reply so far and they question what type of rear axle I have.  I assume they are making a reference to the gear ratios.  I referenced Chuck's Chevy truck site and they reference 3 different gear ratios relating to engine size with transmission type used.  But none of the ratio types indicate a 379 cubic inch V8/TH400 transmission.  All I know that it has a 14 bolt differential with 8 lug hubs and dually wheels.  My question is how do I determine the gear ratio that this is?  Also how much should I expect to pay for a replacement rear end?

[Blue 82]

You can look in the glove box for the RPO codes

G80 - Locking Rear Differential

GT4 - 3.73 Rear Axle Gear Ratio

GT5 - 4.10 Rear Axle Gear Ratio

HC2 - 2.76 Rear Axle Gear Ratio

HC4 - 4.56 Rear Axle Gear Ratio

HC8 - 3.21 Rear Axle Gear Ratio

HE3 - 3.07 Rear Axle Gear Ratio

HO4 - 4.11 Rear Axle Gear Ratio

[Blue 82]

Here's some more

4x2, SRW, 3.73 ratio (opt GT4)
4x2, SRW, 4.10 ratio (opt GT5)
4x2, SRW, 4.56 ratio (opt HC4)
4x2, DRW, 10 bolt cover, 3.21 ratio (opt HC8)
4x2, DRW, 10 bolt cover, 3.42 ratio (opt GU6)
4x2, DRW, 10 bolt cover, 3.73 ratio (opt GT4)
4x2, DRW, 10 bolt cover, 4.10 ratio (opt GT5)
4x2, DRW, 10 bolt cover, 4.56 ratio (opt HC4)
4x2, DRW, 14 bolt cover, 3.21 ratio (opt HC8)
4x2, DRW, 14 bolt cover, 3.42 ratio (opt GU6)
4x2, DRW, 14 bolt cover, 3.73 ratio (opt GT4)
4x2, DRW, 14 bolt cover, 4.10 ratio (opt GT5)
4x2, DRW, 14 bolt cover, 4.56 ratio (opt HC4)
4x4, SRW, 3.73 ratio (opt GT4)
4x4, SRW, 4.10 ratio (opt GT5)
4x4, SRW, 4.56 ratio (opt HC4)
4x4, DRW, 14 bolt cover, 3.42 ratio (opt GU6)
4x4, DRW, 14 bolt cover, 3.73 ratio (opt GT4)
4x4, DRW, 14 bolt cover, 4.10 ratio (opt GT5)
4x4, DRW, 14 bolt cover, 4.56 ratio (opt HC4)
4x4, DRW, 10 bolt cover, 3.42 ratio (opt GU6)
4x4, DRW, 10 bolt cover, 3.73 ratio (opt GT4)
4x4, DRW, 10 bolt cover, 4.10 ratio (opt GT5)
4x4, DRW, 10 bolt cover, 4.56 ratio (opt HC4)
4x2, SRW, 3.42 ratio (opt GU6)

[Blue 82]

I've had pretty good luck locating parts here:
http://www.car-part.com/

[Irish_Alley]

its probably 4.10 or 4.56 when you look at the pinion yoke does it look like you can take like 6bolts off like that? if so you have a 10.5" ring gear if you don't see any bolts its a 9" ring gear next question
does the hubs look like semi floater
or
full floater.
with out the brake rotors of course. it seams the 10.5" full floater are more than likely 4.10 at least with my experience. but i would pop the cover and look at the numbers should be dana some more numbers then the last 3 sets you use the 3rd and 2nd from last of that set should be something like 55 11 ?? you just take 55/11= about 4.10