Either a fouled diff or bearing. Probably. Check your diffs against the manual to make sure that they are assembled correctly. The xxx4g+ will encounter torque steer. But it's not like a shaft car, it will make the front have more traction b/c of the way the motor is spinning. it will not make the car go to the left or right. Also make sure your diff is tight, and that the pressure plates are not worn. To check the bearings, put them on a pencil and turn, if there is any drag, you need a new bearing. Look at your axles and make sure that they are not damaged. take them apart and replace any worn parts you see. Make sure that you look at all four corners of the car, because of the diff action.