I found the sticker in my center console and tried looking up the rear end, but all of the sites I found list the rear end codes as starting with an 8 (I have an 85 IROC). None of the codes listed on the sticker, so I'm still kind of lost.

Is there another code I should be looking for? Thanks.

 

Have you looked at this site?
What gear ratio do I have in my GM or Chevrolet rear end, do I have positraction - Drivetrain.com

 

No, that one was new - and that's exactly what I was looking for. According to my sticker, the codes G80 and GU6 are there. So my car has the limited slip posi with 3.42 gears. Thanks a bunch!

 

Look for the letter G.
GU2 2.73
GU5 3.23
GU6 3.42
G80 limited slip
G92 performance gear ratio?? I think 3.73's

 

And all this is assuming in the last 26 years no prior owners have messed with the car, and changed the rear gears. Just do a search here and find how many guys want to, or have changed their gears. I'll bet if my car had a "sticker", it wouldn't show it has a Ford 9" with 4.10 gears!

 

Well, if it were reving 4k at 70 it definitely wouldn't be a GU2 anymore...

 

That's a good point. I was told that the rear end hadn't been switched, but I guess I don't know for sure. Is the gear ratio or code stamped someplace on the actual unit, or do I need to tear it apart to find out for sure?

 

you can jack the rear up and mark the drive shaft and tires. count how many times you turn the driveshaft to make one full revolution of the tires and thats your gear ratio. put it back in park and try spinning one tire. if it doesnt spin, you have a limited slip. if the wheel turns and the other side spins opposite, you have an open rear or a badly worn out limited slip

 

Did you see any RPO's that had a G? If it was a TPI it probably came with 3:23's. I know it's not like you bought some 69 Camaro with an engine hanging out so far that you can almost touch heaven with it. It's harder to tell if your gears have been swapped.
I know with 2:73's you will rev about 1800 @ 70 mph.

 

This way only works on a limited slip rear. With an open diff, you leave one wheel on the ground, and turn the free wheel two revolutions instead of one. The limited slip method above does turn the wheels two revolutions as well, one revolution on the left and one on the right=2. That's how the differential rolls!
But, if you have no maintenance history on the car, the best method is to remove the diff cover, clean everything out, inspect the gears etc., and change the oil. Oh, and while you just happen to be inside, check the gear ratio. The ring gear usually has numbers stamped on the edge. Divide the small number into the big number, and that's the ratio. Or, you could also count the ring & pinion gears and do the same math.