That's very early for a diff. They are fairly strong unless you do a lot of burnouts in the ice and then maybe a spider gear.
But if it leaks and gets low you usually notice the diff making noise or hum/howl.
A used diff can be found for $250 fairly easy. It's about 2-3 hrs labor at most from an independant shop.
On a side note. If you have SSG(aka SMG-1) then really be sure you know what's to expect when that transmission pump goes out. It's an expensive fix and recently a dealer charged a guy $8K to fix.
OEM CSL WHEELS - OEM CSL BRAKES - OEM AUTOFOLD MIRRORS - CSL TRUNK - CSL DIFFUSER - AA SPORT TUNE - AA PULLEYS - BILSTEIN PSS9s - AUTOSOLUTIONS SSK
CF CSL RACE LIP - NAVI HU - BMW PERFORMANCE CF STRUT BAR - BMW GROUP-N MTRMTS - UUC SS BRAKELINES, TRANS MOUNTS, RTABs - 4.10 BMW M-SPORT DIFF