I took the car to the mechanic to get my front drive shaft U-joints redone. He appeared to be concerned about the differential locking seal, and he showed me some oil in that area. I know very little about what the seals ought to look like, but he indicated that it is a leak. Do these routinely go bad, and what is the solution for taking care of this problem?

2004 Disco, 62K miles. In the process of doing the 60K mile service.

 

Is it dripping on the ground or just a wet seeping spot on the t/case?
Seeping I don't worry about, just need to check the fluid level every 4 to 6 months .