Hemicat, if you are not sure it has been changed then probably not a bad idea to do it. A couple of tips.
Go to your GM dealer and purchase two "crush" washers. The diff in mine leaked when it was new and as I found out the mechanic re used the original washers. They continued to leak. The washers are stupid expensive.
Purchased Mobil One gear oil with the additive already installed. They came in quart bottles that were threaded and I used a pump for changing outboard gear oil.
With the car securely up in the air and supported with jack stands it is best to try to keep the car as level as possible. Drain the fluid out.
Used a large bowl with steaming hot water. Set the gear lube in it, the heat makes it easier to pump the lube in. Don't let the hose off the pump fall into the housing. If you do just put the car up for sale.
As you know the lube stinks so you can't use it for fish and chips.
I took it to a local shop and had it done today. I presume that fluid has been in there for 16 years so time to change it. They said the old fluid still looked pretty good but I changed them both with synthetic fluid. Should not have to worry about it for another 5 years or so.