A '99 with a G80 will have the Eaton Gov-Lock rear locking differential. Under normal driving conditions, it behaves like a normal open differential. When vehicle speeds are below ~20 mph and one wheel spins ~100 RPM faster than the other, there's a mechanism in the differential that locks the differential to send power to both wheels. It's a true locker, only it's kind of opposite of other "automatic" lockers. Other lockers are normally locked, but have a mechanism to allow for differentiation when cornering. The G80, on the other hand, is normally open, but locks up under certain circumstances.
I know I've seen it before, but I can't remember where it's at, but there's a short video somewhere on the internet that shows how it works. Put "Eaton Gov-lock" or G80 into your favorite search engine and you'll find lots of hits.
It also seems to have a bad reputation on the net, especially the 10 bolt version (which is what should be in your Tahoe). Some attribute it to "abuse". Where the mechanism "locks" it together after it's started slipping, it can be pretty "harsh" under the right conditions, which probably contributes to it breaking, especially in the smaller 10 bolt rear.
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