Yeah, the rear diff has a check bolt on the left side of the diff, down near the bottom. Take the check bolt out and fill through the top oil fill cap, until the gear oil is up to the bottom of the check bolt hole. The drain for the rear diff is on the bottom, right side under the skid plate.
The front diff is different. The oil fill cap is down lower on the side of the diff. On the front diff, just pour the gear oil in the oil fill hole and when it comes up to the bottom of the hole, it's full. Of course, the drain plug is on the bottom of the front diff, as well.
Both diffs use hypoid SAE#80 gear oil, any brand will be fine.
Most 4-wheeler problems are caused by a loose nut connecting the handlebars and the seat!!
You only need two tools in life -- WD-40 and duct tape. If it doesn't move and should, use the WD-40. If it shouldn't move and does, use the duct tape.