I am seeing the same thing. To be honest, I used a whole tube of Trans-X (7oz) in 2003 when I got my T/A because the rear chattered and shuddered around corners. Had no issues whatsoever with that quantity. Many claim "too much" causes slipping, and maybe that's true, but I guess it wasn't for me, or maybe 7oz isn't "too much".
My diff cover began leaking about 2 years ago, removed cover, drained, cleaned out remaining oil, cleaned magnet, flattened holes on diff cover, installed new gasket, bolted up, filled, added 7oz of Trans-X, and all is good. I can break both wheels loose at 45MPH, if there was going to be any slipping in any component of my drive-train, it would manifest under that load. ...but I go sideways with two smoking tires, not straight with only one. My opinion is 7oz of Trans-X is fine. Nearly twenty years on and I have no issues with my rear end. Now, that is not to say that Trans-X is "just as potent" as all other brands. Maybe it's the weakest and 7oz of GM's brand or someone else's *is* way too much.
Go to the dealer and buy their "bottle". If your car is 2oz, that's what they will sell you. If it needs 4oz, that's what you will get. Should be around $10.
Also read that some synthetic oils like Amsoil have the additive in them already. If you chose to go that route, verify that whatever product you buy does indeed have those additives.
EDIT: Looked over at shbox.com He has a GM 4oz bottle pictured sitting on (of all things) a 1996 service manual. I'd say 4oz is the answer. http://shbox.com/1/lsa.jpg
Conflicting info is hard to get through. Maybe they initially used 2oz but later found 4oz was required and released a service bulletin. Pretty sure our manuals advise against synthetic oil in the rear end, but if you go to Chevy now, they recommend (and sell) synthetic for back there. Times, technology, and formulations change, as such, so do the recommendations. Looking at "general recommendations" from LSD manufactures such as Yukon, they call for 4oz to be used with 3-4 quarts. I think our diff is 1.5-2 quarts, so 2oz sounds about right by "general standards". However, GM's bottle is 4oz and is not marked in such a way as to gauge what half a bottle is, so I assume GM expects us to use the entire 4oz. Since this is so confusing, my advice would be to drive to (or call) Chevy and ask a Service Tech how much they use in a '96 Z28 because there is no straight answer to be found...