whats not smooth, is it notchy or bind when turning the diff (like a high spot)?
1st - assuming you have no defective parts, it could be the outdrives themselves. its made out of plastic and can distorted.
2nd - you could have damaged the diff by tighting it without allowing to seat as its tightend.
also fit and finish is very inportant. ensure that the thrust bearings/spring are seated on the diff half and the locking nut is fully seated and flat...