how do i know if i have a Ltd slip rear end?

You have to check your RPO code listings. My service parts identifier label containing the RPO codes is located on the inside of the door to my glove box.

I'll get the codes that specify the rear end setups in a bit here.

*EDIT* - Here they are for a 2000 and probably more

F46 Chassis: All Wheel Drive
GT4 Rear Axle: 3.73 Ratio
GT5 Rear Axle: 4.10 Ratio
GU4 Rear Axle: 3.08 Ratio
GU6 Rear Axle: 3.42 Ratio
G67 Level Control: Automatic, Air
G80 Rear Axle: Positraction, Limited Slip

The G80 should be the same for all.

Does GM put limited slip in the rear only? Or do some models have it in the front too.

second question. What is the size on the front ring gear? 5.25"?

Only the rear axle.

7.25" for the s-series.

Ok, I think this information is accurate:
Front axle 7.25" ring gear 26 spline axle
Rear axle 7.625" ring gear 28 spline axle

I have found some conflicting information about the rear axle specs (some say it's an 8.5" 30 spline after '95...)

thanks