I am currently working on a 2002 yamaha big bear 400 with 4x4. I was driving it after running through the trails with my buddies and i was running on gravel road to get back to the house. While riding back the 4x4 started throwing itself in an out of 4x4. It's not an electrical issue. If I drive it it will pop in and out by itself and I need to know what might be the issue.

I have the front differential out of the atv right now and I am started to tear it apart.What might be the issue?
also does anybody know where I can get a pdf for my atv or a diagram for dissasembly of the front diff.
Thanks!