I was working on my 1999 Blazer ZR2 last evening changing the rear u joint on the driveshaft and while the drive shaft was off of the car i also replaced the transfer case seal and oil (Auto-Trac 2)

While i had the car on the lift i could clearly see that it is rust on the diff cover on the rear axle. Actually so much rust that i could see that there have t be a pin size hole in the cover so i need to replace it

Does the ZR2 have a 8.5" rear? Having said tha will i then have to order a 8,5" cover?

I called the parts firm i usually use and they told me they have 3 different diff cover for the 1999 Blazer so i need some help to ID the rear axle on my 1999 Blazer ZR2 so i will order the correct cover

So i need help to identify the rear axle on my Blazer ZR2 so i can order a new CORRECT diff cover for the rear axle for my car. I do not want to use the car before i have replaced the cover

Since the ZR2 is a "special off road" Blazer which is higher and wider compared to other Blazers i suppose the rear axle is a limited slip?
The cover is fastened with 10 bolts if that matters?

I went out in the car and took a couple of pictures of the optons list in the glove compartment so if any here can help me find the correcr cover for the rear axle on my Blazer ZR2 it will help me a lot

Below is a picture of the cover on the rear axle and also a picture of the options list in the glove compartment: in my car. See the red arrow pointing towards option G80 if i am correct in hat G80 stand for Limited Slip?

Here is a picture of the rear axle cover where you can see oil is seeping out and on the cover

You can see that the oil is on the cover on the rear axle so there have to be a pin size hole in the cover


Below is a picture of the options list in the glove compartment on my car and a little red arrow is pointing towards option G80. Is it correct that option G80 stand for Limited Slip differential?

 

That is a 8.5, 10 bolt rear end with a GT4 3.73 gear ratio and a G80 locker. Mine had the same RPO's but had been swapped with a 7.5 open diff with 3.42. But at least definately a 8.5 which is a huge plus

edit: the G80 is not a conventional limited slip differential. It is referred to as a gov-lock. Same basic function but not nearly as dependable and prone to break

 

Nailed it!! As an 8.5 note the stepped-down axle tube size for the S10/Blazer, the very round cover, and the triangular tabs on the bottom of the housing. G80 is lovingly called a "Gov-Bomb" because they can grenade if it suddenly locks up under real power. But behind a V-6 will be fine.

Just curious - are you saying someone put a 7.5 with 3.42 in the rear when your front was still a 3.73?

 

I actually have a '04 Blazer Xtreme.

 

So yours is a 2WD. Is yours a 4.3 5-speed manual? As I understand it those are the only 2WDs that had the 8.625 rears (8.625 started in 2000). The rear in my 1989 S10 is an 8.625 housing from a 2002 Extreme 4.3-5speed. They are like hen's teeth.

Saint, you may want to get the fill plug out before you take that rear cover off. Also would be a great time to replace any brake backing plates that are rusted out.

 

As an FYI, when you go to fill the diff after replacing the cover, use ONLY gear oil without limited slip additives. If you want to ruin the Gov-Loc, make sure you use the additives. I have over 280K miles on my ZR2 and the Gov-Loc still works like it should. A good portion of those miles are offroad, most being highway miles. I have a hunch a lot of folks that have had problems with the Gov-Loc put gear oil with the limited slip additives into their rear diff. I've only used 80W90 gear oil, making sure there is no additives.

 

I will gladly trade you a non G80 rear diff for your G80 diff. I have been looking for one for quite some time, but have not been able to find an 8.625"...lots of larger one, but not this one.
In fact I found and purchased a 7.5 / 7.625" one by mistake.....anyone need it?

 

Very true that it doesn't need limited slip additive, but I don't really see how it would hurt. I think most of the people who have trouble with Gov locks are trying to use the Gov lock to spin both rear tires on dry pavement. This may be over-simplified, but think of a device where a cog is thrown out by centrifugal force to lock things up when the difference in the speed of the two axles exceeds a pre-determined ratio. Works OK to keep you from getting stuck in snow or mud. Not so much when it locks up suddenly under full power when on dry pavement.

 

It says in the owners manual: Axle lubricant use only GM part number 1052271. It also says: Add no additive.
I find this strange since it is a Limited Slip differential.

So i have been reading a bit about the oil and if i should use additive but there are so many differents answers about this on the internet

Should i use synthetic 80W-90 oil without any additive or should i use mineral based GL-5 with additive?

 

There is a great how to post that covers this job by Swartkk. I could look for it but so could you.

I used it when I had mine open to change the rear axle bearing..