I agree, you'll have to pull the side cover off. First thing I would do is unhook the conrol arm from the linkage that goes to the clutches. It's an allen head stripper bolt, and will be obvious once the cover is off. Make sure the control arm moves freely, these have grease fittings (or should) on the operator side, but are often never greased. If there is no binding there, you'll have to check the clutches and thrust bearings. It does seem odd that both would go at the same time, but things like that DO happen. If it's like the 600, the other thing to check is make sure you are getting hydraulic fluid to the clutches. Have to start it with the side over off, but you should be able to see fluid coming from the clutch packs, There is a check in the system that allows 20PSI flow to the clutches. While you have it apart, check everything for wear or damage, including each chain link on both sides. I found a broken chain link on mine, and two idler sprockets were completely shot. The chain was still intact, but with one side of a link broken it was only a matter of time before it failed. I also strongly suggest you drag a strong magnet in the sump area. I found a 7" long Craftsman chisel in my 600 that had score marks on it from coming into contact with the drive chains.