If you're brave and saavy there are videos on youtube and other blackberry sites showing you how to remove your trackball and clean it or get a new one and reinstall it.
Other than that, if you're out of warranty on the phone you'd have to pay $50 to get a replacement.
Removing the trackball and replacing it is very easy. I would recommend buying a new one it comes with the ring and ball for $9.99 at the crackberry store.
If you have had it for two years congradulations thats a long time with one trackball. Mine last 6 months tops.