So why won't it slide all the way on, its because the splined hub gear hits this retaining ring on the axle shaft.
Solution 3: Again some people just remove the retaining ring and then install the hub. Doing that will let the hub fully seat. But now the axle can float in and out. At ride hieght I could push my axle in about 5/8" with my finger... at full droop - it might be able to go in even farther and completely seperate from the locking hub which isn't ideal! While it would work - I really don't like this idea of the axle floating around that much.
Solution 4: There is a also small spacer / thrust washer behind the retaining ring. It hard to see the picture, but its there. If you remove the retaining ring, then remove the spacer and then reinstall the retaining ring, you can slide the hub on and have it fully seat. But that lets the retaining ring ride against the end of the spindle where you could possibly damage the spindle or the retaining ring maybe even booger up the threads. Again this will work - but not real ideal.