Not all LTZ models are 4WD, I ordered my 2007 LTZ as 2WD so it was possible and they are out there.
You asked an interesting question so I took it to a buddy who has been a Chevrolet dealer for over 30 years. He was gracious enough to answer the question and to give us a breakdown of the sales numbers from model years 2007 - 2013. Based on records from his five Chevrolet dealerships: Avalanche model years 2007 - 2012 show an average of 71% of the LTZ trim level were 4WD and 29% were 2WD. The BD 2013 model year is totally different as over 90% of those were sold as 4WD LTZ models. Basically the BD around here was fully optioned LTZ trim with 4WD.
Hope you found that interesting, I know I was surprised at the numbers.