I've actually seen this in almost every 5MT I've worked on. It's pretty common, but varies in severity.
It's a pain in the butt to fix though. You need to completely remove the linkage and the attachment point on the gear selector shaft. Then you need to pull the driveshaft, pull the crossmember, lean the box back and remove the tailstock portion with the center diff. once removed, the seal can be removed and replaced fairly easily.