I have a 97 Tahoe 4x4 with 180K miles on it that I have only had for about 6 months. Today, I put it in gear and started down the driveway and it locked up, like I had thrown it in park while moving. It will go into Drive and Reverse, I can feel it engage, but will not move. I can give it some gas and it feels like the brakes are on, engine lifts, it wants to move but can't. I checked the rear diff and it's fine, nothing out of the ordinary so now that has a new gasket and oil. Any thoughts on what this might be? Or do I need to get it to the transmission shop?