Flywheel to crankshaft bolts could have become loose. This sounds like a classic case of that. Exhaust all other possibilities before splitting the tractor to check flywheel bolts. Another cause could be a broken crankshaft. Inclusion flaw in casting, or lugging the tractor down too often, and for extended periods. Sit in seat, rev tractor up until it vibrates, depress clutch pedal, if vibration is reduced, or disappears, then it is a possible broken crank, or loose flywheel. Check everything else as others have mentioned.