Locked up rear end

[Drees's]

Hello, I am new to model A and recently purchased a chassis and motor, I had the motor rebuilt and it is free to turn,( I can turn it by hand with the trans in neutral) but when I put the trans in gear and try to push it, both rear tires lock up and skid along. I did this to see if the engine would move, it does not when in gear. When in neutral, everything rolls freely. Can anyone tell me where to start looking, trans, rear end?