For the "U" , we decided to make a change to that after the first two days based on feedback we received, mentioned Here :

"U: [UPDATED] Starting today, complete the level 65 Vanguard Request, "Hunt Brutal Ovoliths" on the Island of Dawn and you'll receive a U."

As for the "N" , that can drop from all of the following :

Kalivan's Challenge
Vault of Kaprima
Demokron Factory
Demokron Factory (Hard)
Sky Cruiser Endeavor (Hard)
Shadow Sanguinary
Shadow Sanguinary (Hard)
Lilith's Keep
Lilith's Keep (Hard)
Ruinous Manor
Ruinous Manor (Hard)

It is worth mentioning, that I am getting some reports that it's not a guaranteed drop in all of the above listed Dungeons.