904 or rear axle runs jerky help!

If the reear has a limited slip differential - jack the rear tires off the ground and try to turn them with the trans in Neutral. If you feel it binding, that's youre problem. If the rear doesn't have a limited slip have a friend turn the opposie side wheel at the same time to make sure it's a good test. If it's all smooth - it's in the trans.
A broken brake shoe or return spring can give the same issues.