I just purchased a 2009 LTZ and have the same type of issue.
First let me state the obvious.....the navigation system stinks...we'll actually it really stinks

I have 2 Hondas with Navigation that make the Avalanche system look like it was created and thrown in as an after thought, rather than a feature that customers really care about and take the time to do it right.
The visual quality of the system is poor and the screen layout is hard to read and hard to understand. It will announce freeway exits by number not by name. As an example, it tells me to take the next exit 2993C rather than Walnut St. Who in their right mind would know Walnut st is exit 2993C. When I miss a turn and it has to reroute me, I feel like the system knows I am lost and is screwing around with me on purpose to have a little fun. Rather than asking me to make a U turn when it is safe, it will send me off in a spaghetti bowl of turns trying to get me back to where I am now.
Not announcing the street names and just saying " next left turn" is really lame. The whole system in not intuitive not user friendly

While it might sound like I am a Honda biggot etc, believe me, I am only offering a fair assessment based on what I have seen and experienced. For the most part, I love my new Avalanche and was just shocked at how amateurish and lame the navigation system is on this vehicle.