It's teh C clip on the splines. It can be a real pain in the ass. I can tear down the front and pop off the axles in like 10 minutes. Putting in the new ones once took me 3 hours. I used to be gentle and just try to push the things in. Now what I do is get a good grip on the axle and just keep shoving it in and out(sexual inuendo) until it pops in. You'll know when it pops in to. So just grease up the splines and keep like jamming it in untill it goes in. Nothing bad can really happen unless you have horrible horrible luck..