Posi In A 2003 Chevy Silverado! Help!!!!

HEY.... I have a 2003 chevy Silverado with a V6 short cab short bed. I found out my gear ratio by looking at the other forums and finding the code... the one thing that is just really confusing is trying to find a posi... i mean what's all this 30 spine or the 8.5??? i mean i don't even know what my truck has beside the 3.42 gears... can you help a guy out??? and if u can point me in the right direction of buying a posi unit that would be great... also.. what is the difference between a posi unit with a clutch and one with out? thanks
KYLE :)

I've been looking at the powertrax website. They have 2 types, although the full locker looks like over kill for my needs. The good part is you don't have to touch the ring and pinion gears. Have a look www.powertrax.com