Mopar 8.75 posi. wont work. Why are there 2 rows of spline?

Had the same thing happen in my b body. Went from a Eaton posi that had the full spline to the factory sure grip old style 2 sets of splines( lack of what the actual wording of this type is.) Same as you backed it out of the garage, put it in park and watched the car roll down the drive.Hopped in and got it stopped,put it in drive and no movement just a whining sound. Pushed it back in the garage and back apart it came.Called Dr. Diff and he explained to me the cause,and the cure.Like you I assembled this rear from many different parts and the axles were too short for the 2 spline setup. He has the rebuild parts to up grade what you have.Just to let you know the Eaton set up with green bearings was raced alot before the change over so the axles did fine with the single spline set up.