Hi Walt,

It really depends where and what the dirt is. Could the dirt be fungus?

Some rear elements are is sealed/bonded together, and same are tightly glued in.
So on the safe side, Nikon would need the necessary spare parts before the
repair.

Your local repair shop may have a go at the lens, but you must get written
assurance that they won't damage it. Also, what kind of insurance cover can they
provide?

Regards,

Sover