When I engage the blades, The whole tractor jerks pretty hard in a way, should I say( like a missing tooth on a gear) Seems to do it more while turning or going up hill. When I disengage the blades the jerking stops and the tractor acts fine, as long as the blades aren`t spinning. Pullys seem(idler) to spin OK. Spindel bearings are good, and no loose or broken bolts. Blades aren`t bent and are sharp. Could it be just a belt? Belts and rider are 3 months old.

Yes, the belt can cause vibration, you say the blades are sharp, but are they balanced?

It does sound like you are missing a chunk out of the belt. Carefully inspect the entire belt that goes to the blade pulleys and the idler and tensioner. Remove if you have to.