You have a G80 unit, it should say G80 on the SPID sticker in the glovebox, along with GT4 which is the RPO for 3.73 gears. It's an Eaton Gov-lock unit, not really an LSD. It's basically a locker with a speed governer to keep it from kicking in if the tires lose traction at speeds over ~20 mph. This is done so that it won't engage at higher vehicle speeds, and possibly get both rear wheels spinning. When that happens, you don't have any lateral control, where-as if you are one-wheel-peelin', you still have one tire with lateral traction.. The engagement is very abrupt when it kicks in, though there are some clutches in there to soften it a bit. I just put one in my 94 Silvy (you can put an 8.6 unit in an 8.5" housing if you get the right bearing kit), and compared to an open diff, it helps in low traction situations an incredible amount.
You can do some tweaks to it. If you want the governer to kick in at a higher speed, you can remove weight from the governer weight. Cutting most of it off will dramitically limit when the governer prevents engagement, but that also means you can break both tires loose and get squirrely at higher vehicle speeds. Also, it also allows engagement with a bigger split in wheel speeds, and that might lead to a higher chance of axle damage from the torque spike the axle that's getting engaged would see... hillbillied differentials (missing teeth,) might ensue..
You have an 8.6" ring gear BTW.. They are not asking for ratio, but the diff size i.e. 8.6" 10-bolt, 9.5" 14 bolt etc to determine the cover gasket size. But, RTV does work fine as well.
If you have clunking going on when getting on/off the throttle, you might have excessive lash in the ring and pinion, or the u-joints are worn. On a related note, do you have a PCM tune done on the truck? There are controls on the DBW trucks to help with driveline clunk as well.
