Autozone is one of many sources. I bought my 14mm hex key from a local industrial hardware supply shop ($4 or so).
Changing the diff fluid is just good ol' preventative maintenance.
I can tell you from experience with my former daily driver (a Trans Am GTA with a limited slip diff) that the fluid does wear out. That car had a bad clunk in the rear when going around a tight left corner. A little research (a lot of worry) later, and I found out that this was a common indicator that the diff fluid is tired. So, while it may be a different make, the fluid experiences the same environment and does not maintain it's characteristics indefinitely.
It's a 20 minute job every few years (assuming you can get the plug out.) Well worth doing.
Experience is something you don't get until just after you need it.