Small Engines (Lawn Mowers, etc.): mower deck vibrations, deck components, mandrel

Question
I just changed the left side star mandrel assembly 0n a Craftsman 18.5hp 42 inch cut garden tractor, due to a sheared off mandrel housing screw causing the blade to cut uneven. I am sure I reassemble everything correctly, re-attached the mower deck to the mower.  Everything was good for about 20 minutes of mowing then the mower deck starting vibrating so much I had to stop mowing for fear the deck would shake apart.  I took the deck off and re-inspected everything, still very bad vibration.  Any ideas?

Answer
Was there any damage to the pulleys?  If so, did you replace them?

Is the idler pulley bearing smooth with little play?  Did you replace the belt?  I have had belt wear after replacing deck components.  I counldn't see the belt wear, but a new one solved the vibration issue.
Let me know if this helps.
Eric