Our local mechanic ruled out all options and determined transmission needs to be rebuilt. Costly fix approximately $3400. What causes a transmission to fail like this? Was it something preventable? We always take to Hummer dealer for the required proactive maintenance timely.
I would take the car to a transmission specialist and get a second opinion, you are talking about a very complex and expensive transmission, with out doing several hands on test there is no way of knowing what fault may be affecting the car, you can start with a transmission scan, this may offer up some fault codes as well.