You want the screw that holds the clutch bell on to be nice and tight, but it has to be shimmed just perfect to allow the clutch bell to spin freely. If you physically pull/push the clutch bell back and forth there should be just about half a millimeter or so of play. More or less and it will start to bind. If this is all perfect and you still face problems, make sure you've seated the engine properly and tightly. I had the engine mount move on my XTM once after a not-so-hard smack and everything went out of kilter and started to bind. If it's still a problem, then i'd suggest looking at the diffs and all the bearings in the diffs and the wheel hubs.

Do you have any close up pics of the area you think could be causting the problem? That could help us quite a bit.

Good luck.