I know if the inside set of splines aren't inline with the outside set on the passenger side it won't go in. When I carried my diff to Kelly (kellylmann) he told me when I got it back from him everything was inline and ready to put axles in. His brother told me not to baby it. I was worried I was gonna mess something up going easy with it. Getting pass the first set of splines was easy but then getting it to lock on the second took me a few tries. You should have maybe an 1/8" gap between dust cover and diff. If it's all the way in and locked you can grab in between cv boot and dust cover and pull on it and it shouldn't come out. If it does then it didn't lock in place.
Sorry if this wasn't helpful, I don't know what else it could be.
Nathan