In my opinion, its not really that hard to split the cases. Make sure you keep all the parts organised really good in a big tray of parts, makes it MUCH easier!
The only special tools you'll need are a flywheel puller, about $10 at the dealer, and I made a puller (more of a "pusher") for pressing off the left hand cover off of the crank bearing. It was a 2x6 block of wood, with a hole in the middle (lines up w/crank) and 5 holes that match up with the ones around the magneto. Bolted the block up to the case with extended bolts. Then put a large (M12 or M10) bolt through the hole, up against a nickel siliconed to the crank as to not damage it. A large fender washer went over the bolt on the other side, then held a nut against the washer. Holding the nut and turning the bolt presses against the crank, pulling the case off with a few loud snaps.
Once the case is off it should be pretty easy to see what the problem is. Shifter forks on the KDX seem to wear out/bend pretty easily, take a good look at that. Look at all gears, see if they are siezed to the shaft/bushings, look at drum shifter/slider pins etc.
Puttin the case back together is easy, just slap all the stuff back into the left case first I think, then line it all up into the right case. Then put a large socket over the left side crank, put the magneto bolt through that and tighten it down (make sure you use some yamabond to seal the cases back up FIRST, I forgot to do that once!).
Not too hard over all, just takes some time. :thumb: