Do the test that I suggested earlier and also try holding the spur gear still and turning one tire. If the other tire rotates the opposite way and it feels smooth then the diff should be fine.

I once forgot to put one of the small washers back on that go on either side of the brass spacer behind the slipper spring...that'll make it slip lol. Also make sure you're not missing one of your spur gear screws(this happened to me once also lol).

It's always best to check the simple stuff first that could be overlooked than to go tearing into something like the tranny.