I think that Azog was meant to be an Uruk of Mordor in the books. Why?
First of all, we have this quote:
Quote:
Then Náin stood before the Gate and cried with a great voice: ‘Azog! If you are in come out! Or is the play in the valley too rough?’
Thereupon Azog came forth, and he was a great Orc with a huge iron-clad head, and yet agile and strong. With him came many like him, the fighters of his guard….
We know that Orcs were generally shorter than men, and Azog is referred to as great, like Uruks.
In addition, Sauron had sent his creatures to populate Moria 300 years earlier and this was years after the first Uruks came out of Mordor. Also, Uruks from Mordor are seen in Moria by Gandalf just outside Balin's Tomb.
There are several mentions of Uruks taking command of a group of lesser orcs. In fact, I'm not aware of any orc captain (in the Third Age) that seems to be part of a lesser orc group. Gorbag, Shagrat and Ugluk are all reffered to as large and strong orcs (and orcs is in the books not a specific word as it is in the movies - orcs is the general word and also includes Uruks).
For these reasons, it makes sense to me that Azog, the large commander of the orc armies, was in fact an Uruk in the books.