Heres a tip on replacing diff fluid.
 

I replaced my manual trans and diff fluid Saturday with Motul 300.

First off, what a huge difference in shifting. About 50% of the difference was immediate, 30% in the first 5 miles of driving, but another 20% took a couple hundred miles to appear. I suppose the additives bonding to the metal takes a little time.

As I expected, the old fluid in both the diff and the trans was fairly grungy for only having been used for 9k miles. People complain about the smell, and it did smell, but its didn't seem as bad as most gear oil I've experienced.

Anyway, heres the tip. There was a fair amount of metal on the drain plug for the transmission, and an incredible amount of metal on the diff plug, just as shown in this how-to. For kicks, after all the fluid was drained, I put the drainplug back in and removed it again. Low and behold, it was again completely covered in metal shavings. I did it a third time, and got a small amount more. If I had just replaced the plug and refilled, I'd still have a ton of metal swarf left in the diff housing.

I thought about refilling the diff with cheap fluid for 20 miles to flush the old fluid out before refilling with motul, but decided against it. Im still really glad I happened to put the plug back in.