auto 4wd won't shift back in 2wd

Hello, Dave here, contributing to RepairPal and providing expert advice since 2010. I'm sorry you're having troubles with your Avalanche. I'm wondering - what testing was done to suggest that replacing the transfer case was the best choice? Also, where did you source the transfer case from? Is it remanufactured? Used? From and for this exact model and configuration? Sometimes there's a guess at diagnosis, and sometimes there's a guess at application of a component. As a result, there are many variables in play here and it's difficult to suggest a course of diagnostic testing going forward - except to verify application of the replaced part first. Seek guidance at a shop near you. https://repairpal.com/auto-repair-near-me Click the link or call us at 877-913-1554. We'll gladly find you a certified RepairPal shop in your area.