Where can I look at gear oil levels, and tweek them as needed? The same question for brake fluid.
For the brake fluid, you check it at the master cylinder on the handlebars. There should be a sight glass, either on the front side of the master cylinder or on the back side. Just look through the sight glass and make sure it's up to the top of the sight glass.
The front diff should have a fill plug. Take the fill plug out and fill it until the gear oil is even with the bottom of the fill hole. I'm not sure about the rear diff on the Rancher 400. It could be the same way as the front diff or it might have a check bolt on the side and fill plug on the top. That's how the 350 Rancher is, so I would say the 400 is the same. If that's how it is, just unbolt the check plug on the side and fill it through the top fill plug hole until gear oil runs out the side check bolt. If it has the check bolt, the check bolt will look light a regular bolt with a washer behind it. I can't remember for sure, but I think it says "Check Bolt" above the check bolt. The fill plug will be bigger, about the size of a half dollar.