[tp_smith]

You could put a little grease on the axle splines and insert them. Pull them back out to see how much contact you have inside the rear end. This will confirm they were really inserted into the gears.

Are the bearings pressed all the way on the axle shaft?

I don't know if the 12 bolt has different axle offsets. If it does, then this could be the problem.

 

[tp_smith]

My retaining plate fits around the large diameter part of the seal where the top arrow is pointing. It does not fit the smaller O ring area. It looks like you need a bigger hole in the retaining horseshoe ring.