So I finally crawled underneath because I wanted to find out what rear end I have, I knew it was a 3.73 but I wanted to know if it was posi or not because I heard it only came with rear discs but I have drums. Found the axle code 6kmco451, looked it up in the Haynes manual and 6km says posi. I put the whole car on jack stands and I tried spinning one wheel and the other wheel didn't move. Turned the car on and put it in gear and both wheels spin. Safe to say its a posi unit? Car is a 84z28 h.o. 5 speed.

No. Only way to be 100% sure is take the cover off and look. If it's a non-worn out posi the other wheel should turn in the same direction as the one you turn. Open would turn in the opposite direction.

I agree, the only way to know is to remove the cover and look inside. I've been fooled too many times by a posi that was worn out, or a bent axle tube, or a galded cross shaft. There is NO OTHER WAY to be sure of what you have than to remove the cover and look at it.

So I found out it is posi, but might be worn out. It engages at first then I get one wheel peel. Or is this how the posi used in 84 works?? Would changing the gear oil bring it back to life? Also what kind of gear oil should I use? I hear posi units require special gear oil?

It's probably the Auburn and it sounds like it's shot. There is no bringing it back to life. At this point it's scrap metal. Replace it with a new Eaton Posi or Yukon Dura Grip. The oil is not as important as the oil additive. Us GM limited slip oil additive only. Don't use any oil additive that the auto parts stores sell.

^^what he said

being an 84 and assuming its a factory rear, when looking for a carrier you need a 26 spline axle count for a direct bolt it.

Post a pic of the unit for verification. You never know if it's been replaced at some point.

Or get a Zexel torsen (Same diff as in the 4th Gen SS's and T/A's) and new axles with 28 splines. This is what I did.

Either way, I'd get 28 spline axles if you can afford them. Much stronger and came factory in the later 3rd gens (89-92).

I can keep the rear end and gears right? Or do I have to get a whole posi unit with gears?

You can keep the gears, the actual ring and pinion but to get the posi unit "fixed" you'll need to replace the differential.

Alright

You will have to reset the backlash and bearing preload. Read the sticky on FAQ about 10 bolts for more information on installing the differential.