why when i put my car into gear only one of the axels turns

Hello, how are you seeing this? Are you checking this with the vehicle in the air on a lift? The reason I ask - i'm just not sure how you could determine this if you didn't have it in a controlled environment like on a lift in a shop. Is the vehicle not driving as it should?

*With vehicle resting on all four tires* .. IF you can actually see one axle shaft rotating but the vehicle is not trying to move - that is indicative of a broken outer cv-joint or stripped splines on the rotating axle.... To be safe be sure to apply parking brake! .. Hands-on visual as well as manual inspection is required by qualifying personnel.