904 2nd gear stucking? Pls, help!

Since you say it seems to lock up when it shifts into 2nd then works in 1st when you stop it sounds like the rear servo is stuck or it broke and jammed in the bore. Also possible you cranked down the rear band too tight. Jack it up and run it with the tires in the air with it in manual low then shift to 2nd gear. If it locks up the wheels the rear band is being applied for some reason when it shouldn't which could be the rear band adjustment or stuck/broken servo.