2 things.... First what type of oil are you using when you change your oil? Second.... if you jack up the rear of the quad and properly support it, start it up put it in gear will the rear wheels move then? if so that would be no load and your clutches are either out of adjustment or burt up. if it dosen't move then you may have either sheered an input shaft or broken the drive shaft u-joint.... Did this happen all of a sudden? and what were you doing when it happened?
automotive oil!!??? if so there is your problem. These engine and trannys are combined and require a molly free oil for the clutches. conventional motor oil will deteriorate the clutch fibers. I going to say you toasted your clutches.
Good news is that you can get new ones for not that many georges.