I would guess the rear brake assembly also, I know mine gets really noisy after riding in sand or fine crushed stone. I find flushing the assembly with a hose with a high pressure nozzle usually stops or slows down the grinding until the crud builds back up again. From my reading this is a common issue on utv's with similiar rear brake systems.
I would still do a thorough check of the other components mentioned previously just to be sure. Once I get mine cleaned out, it stays quiet while riding in my yard or on the road, but it does get noisy quickly when I get offroad into gravel, sand or other course materials. Had my rear brake sieze up once, by debris getting into emergency brake cable connection at the rear, took a while to get that freed up!