What breaks with the G80 can't usually be fixed because it actually fractures the carrier, so in the face of spending 500 or more dollars on a carrier, people usually opt to go with a wrecking yard open and rebuild that, or upgrade to something like an Auburn Gear LSD. Depending on where it breaks in the carrier sometimes it spits out the regulator and lock device, sometimes it spilts the carrier enough down the middle to where it spits out the spider gears and cross shaft. If it does option 2 you'll know it when the truck stops moving. Option one often leaves you able to keep on driving like as if you have an open diff, sometimes forever and then sometimes the carrier finally gives out and leaves you with a locked up rear end that won't rotate and most people do not realize they had a problem until they go to change the gear oil and find little springs and metal pieces of weird shaped doo dads in the bottom of the diff.
If you absolutely need a LSD, then moving on to something like an Auburn LSD, which uses a spring packed clutch and is pretty reliable overall is the best compromise overall. The next level above that is a detroit locker but I hear a lot of folks complain about the ratcheting on the rear end when they make a turn, and then the upper tier is something like an ARB air locker, which gives you the ultimate ability to have super nice road manngers, and a locked up diff when you need traction across the whole axle.
I think the quantity for gear oil is just a tad less than 2 quartz.
I know when I had my Ram quadcab it had a posi in the rear and I hated it on rainy days. It actually contributed to less traction because one wheel was already breaking traction so the moment you tapped the gas the rear end would get out of control fast.