posi vs. open diff on a box Caprice

I've got an open diff on my '87 and want a posi. I found a '87-90 (not sure which year exactly) with what I believe is a posi at the local junkyard. Is there any difference between these rear ends other than the gears? What I'm wondering is if I should even bother getting the whole rear end from this posi car and having it checked over and possibly rebuilt or should I just have my current open diff rebuilt and converted.

edit- Also, the codes in the trunk are GU6 and G80. Was I correctly informed G80 means posi? Anybody know the ratio of GU6? And I know you can tell if it's posi by spinning 1 wheel when the car's in the air and if they both turn the same way it's posi or open I don't remember, and if they turn opposite directions it the opposite, whichever that is I don't remember. Need some clarification, thanks!

edit- Also how do I tell if it's got the beefier springs I want?

http://i121.photobucket.com/albums/o219/ernestmayhand/Caprice%20resto/junkyardcaprice01.jpg

I'd like to know that as well...

The above car is a 1990. 10th digit in VIN = K. K = 1990.

G80 = posi
GU6 = 3.42:1

Want beefier springs? Go get a set of 'heavy duty' springs at Autozone. These correspond with the 9C1 "RJ" springs which are stiff. They'll be better than any worn-out junkyard springs. That's one thing I don't like to use from a junkyard - springs.

3.42... thats not bad at all. I've got sh*t 2.73 gears in mine. The rear end is good on gas, but isn't all that great in terms of torque. Even though they're economy gears the car still seemed to have not that much trouble getting going. Then again, I haven't much to compare to. I've been looking for a complete rear end with 3.42 - 3.73 gear ratio & a limited slip differential, but I'm having trouble finding one with 5 x 4.75 bolt pattern. I'd just change it to 5 on 5, but then I'd have to buy 2 new rims and wouldn't be able to rotate my tires.

I didn't even think about the bolt pattern. Mine's got the 3.08 gears in the open diff. And it was a 305 with a 200R4. Any idea what the bolt pattern is?

4.75 unless it's a wagon or a 9C1 (although some 9C1s came with 4.75 as well).

So I guess back to my original question...is there any difference in the housings between an open and a posi or is it just on the insides of the pumpkin?

So I guess back to my original question...is there any difference in the housings between an open and a posi or is it just on the insides of the pumpkin?

The only difference is the carrier. The housings should be the same.

Just make sure you get the proper carrier and be prepared to do the entire setup on the differential once you are done. This is time consuming, and if not done, can result in a noisy, whiney diff at the least, and broken parts at the worst.

I jacked up the rear end on my '87 and turned the wheel with anticipation that the other side would rotate the same direction or the opposite direction. But the other side didn't turn at all. Normal?

Put it in park and try turning the wheel with the car jacked up. Even if you have posi, or limited slip the wheel will still turn the other way, but there will be significant resistance, especially the harder you turn it.

No motor or trans or driveshaft in there right now...

http://i121.photobucket.com/albums/o219/ernestmayhand/Caprice%20resto/birdseye.jpg

Hm, well in that case the driveshaft yoke is spinning with the wheel when you turn it. Sorry man... no posi.

Just pull the diff cover open and have a look at the carrier. An open diff will simply have 4 gears inside, and the large ring gear on the outside. Posi units will be different depending on manufacture - look for clutch plates and if you see any, you have posi.