wobbling mower deck?

Could be a few things. Bad bearing in one of the mandrels, idler pulley has a flat spot on it, belt missing a chunk, bent/way out of balance blade...or if you hit something REALLY hard, a bad/bent spindle shaft. Carefully check all of the above.

Blades are new with only a few hours on them and in good shape. Belt is in good shape. I thought it was one of the idler pulleys and decided to replace them all, they are in new condition, but I will double check for a flat spot. No bent spindle, and all bearings seem to be good but how would I know for sure? Everything seems tight, no wobble in any individual bearing or pulley, all blades turn freely by hand. The spring on the idler pulley seems ok but it is old and might have lost some of it's tension, allowing that idler to move more than it should. Could that be it?

Motor runs good and smooth. I thought it might be in the clutch but it is working fine.

Did you pull the belt when you checked the spindles for side play? If there is tension on the belt you might not feel the looseness.

Yep, the belt's been off many times throughout my investigation. I'm about to go back to the blades. Even though they are new, could they be out of balance bad enough to cause this?

I wouldn't think so, but it never hurts to check the balance. While you have all the blades of, turn the deck on and see if you still have the vibe without the blades on?

Good point, already tried that and the deck still wobbled. I guess it's not the blades. This one has me stumped.

You have the double belt system right, engine to deck belt then the deck has a second belt that drives the blades? If so, remove the belt that drives the blades and turn the deck on. Still wobble?