Is it typical for the splines inside the u-joint on both the output shaft and the drive shaft sides to wear "lopsided"?
it might be. The output shaft end of the U-joint doesn't change angles while it's rotating. But the other end does constantly with every new angle of the rear wheel. Not talking about the yoke in the middle. If you were to be reincarnated into a u-joint which half of a u-joint would you rather be. Just trying to be humourous.