I was looking into the 3.73 pro-street gears by strange and i saw it has 2carrier and a 3carrier. what do they mean by 2 or 3 carrier?

 

Since you're car is an SS with an A4, I'm 99% sure they came with 3.23 gears, which are on a 3-series carrier (the carrier contains the posi / limited-slip part of the differential, as well as that is where the ring gear bolts to). If you have 2.73 gears, then you've got a 2-series carrier. If you're upgrading to 3.73's, buy them to match the carrier that's currently in your car.

 

The ring gear on a 2 series carrier is a little thicker than a 3 series carrier.

 

So a 98-02 z28 a4 is considered as a 3 carrier right?

 

No, not always. The cars using 3-series carriers are 3.23 geared A4's, or 3.42 geared M6's. However, 2.73's are the more common A4 gear, and they are using a 2-series carrier (and they're available through all the years, '93 - '02).