Transmission won’t shift but will shift to neutral

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My 2005 GMC Yukon (2WD Automatic) transmission is not shifting. Background: While driving it felt like it was slipping while driving, pulled over and could only move forward in what felt like 4th gear. After it was towed it only would move forward, it would not move in reverse Problem: The car will not produce enough torque to move forward nor will it go in reverse. It will shift to Neutral and Park, but D,1,2,3 and R all feel and act the same. While in drive the car will only produce 2,000 RPM, while in Neutral it will produce normal RPM with no limit. It’s like the car is stuck in an extremely high gear and has no torque to move. Any idea what the problem is?