I have a 2003 CTS with a slight whining noise coming from the rear differential. I found some "Limited Slip (posi-traction) Axle Lubricant Additive" (GM P/N 1052358) that is supposed to correct this, but it is only for cars with limited slip posi-traction rear axles.

I have traction control, but I don't think that is the same thing as posi-traction. How do I find out if my car has a limited slip (posi-traction) differential?

 

Well, I have an 05 and I can tell you how to find this info. I'm not sure if it's going to be the same for an 03 but lets find out.
First, find the Service Parts ID Label. In 05 it is located in the trunk attached to the under side of the spare tire cover. It should give you a list of all the RPO (regular production options) installed on your car. These codes are two letters and a number or a letter and two numbers. G80 is the code for "Axle Positraction Limited Slip"
Write down all the codes, because I think there is a site that gives you this information for most cars. I'll just have to try and locate it again. Anybody else out there that has it on the tip of their fingers, could you help us out?
If you want post a couple more RPO codes and I'll tell you what they are in my book and you can see if it jives with what's on your car.
Mike.

 

Thanks Underdog. I don't have G80, so it looks like I don't have limited slip. The axle code on my car I believe is GU6.

 

GU6 is the gear ratio, in this case 3.42.