OK - thanks. Any other parts besides the axle and hubs that I should replace while I have it all taken apart?
Check your axle bearings and seals in both sides of the diff and the axle bearing and seal in the brake backing plate (aluminum piece). Those bearings are a common problem with a Fourtrax 300. It's best to check them before removing the axle. Grab the ATV by the back cargo rack and shake it side to side and up and down and back and forth, anyway you can, while watching for any movement where the axle comes through the diff and the brake drum cover. If the bearings are bad, you will be able to see the axle move when you shake it. When you shake it up and down, pick it up off the ground and sit it back down while watching the axle. It's a lot easier if you have someone else do the lifting and shaking while you watch closely.
If the axle moves any at all, it's a good idea to replace the bearings and seals, because it will wear your ring and pinion gears out in no time at all.
If you don't see the axle move any, you shouldn't need anything else.