How do I Identify what rear diff my 1990 full size has? I just swapped out the rear diff fluit and gasket, It diddnt appear to have a posi, that I could see, but I want to make sure I dont have to put in any addative. Is there a code or photo that I can use to identify what rear diff I have. Thanks in advance for any help.

 

All bets are off on something that old in terms of being able to look for a code that would tell you. If you snap a picture of the inside of the axle, I'm sure we'll be able to identify it for you. The axle itself should be a 8.625" GM Corporate 10-bolt.

Alternatively, you can run some Google image searches for "open differential" "GM limited slip differential", and "GM gov-loc differential" and you should find pictures that will help you identify what you have.