i have a bayou 300 2 wheel drive-1999-what oil goes in the rear differential and how much --and I need an online diagram about putting the gear change mechanism back together

 

Lots of websites have online diagrams. Try bikebandit.com or ronnies.com. Older models (86-87) uses API GL-5 Hypoid gear oil SAE 80 or 90. Fill to the bottom of the opening 0.3L. Remember this is an older model, not sure if your's is the same. Oil type used to be stamped on the diff near the fill plug.