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.
OE CSL WHEELS - OE CSL BRAKES - OE AUTOFLD MIRRORS - CSL RACK - CSL TRUNK - CSL DIFFUSER - AA TUNE - AA PULLEYS - AS 40% SSK - BILSTEIN PSS9s
CF CSL LIP - NAV- BMWP CF STRUT BAR - BMW GROUP N MNTS - GRUPPE M CF INTAKE - SUPERSPRINT - UUC S/S LINES, RTABs - H&R SWAYS - 4.10 M-SPORT DIFF