Adjust the blade belt - Craftsman 42 in. Deck 19.5hp Lawn Tractor

Sounds like the belt might be smaller/shorter than the required belt, not providing enough clearance to the spindle pulley(s) when tension is release. Might verify correct routing of the belt as well.

Your owner's manual should tell you

Depending on which model you have, you will have to loosen the belt adjustment by the gear drive mechanism, then slip the belt over the drive pulley. This should allow enough room to get the belt over the blade and off. It might be easier to remove the blade too.
Then install the new belt over blade first then over drive gear box pulley, then tighten adjustment for the belt at the gearbox.

Remove blade drive belt. turn each blade by hand. Sounds as if you have bearing problems. Each spindle has an upper and lower bearing of the same size. They are notorious for going out! You can get them at most any auto parts store. Also check the tensioner pulley bearing. They go out quite often. As for the jerking, make sure the two drive disengage rods are fully in. Check tension on drive belt and belt. These belts get worn quickly. Be sure to blow the trash and dirt from your mower after each use. Especially the Hydraulic motors and the mower deck. This will save you lots of trouble. If you wash the mower, be sure and run the blades and Hrdroes to get water dried from bearings and pulleys and belts.

Assuming you have put the blade drive lever into 'disengage' check the blade drive belt tension. With blade disengaged, belt should have significant slack. You may need to adjust the control cable, adjustment is usually on the mower deck; or you may have some debris on the deck preventing the pully to release belt tension

check your pulleys to make sure they haven't split as this does happen. Adjust the belt tensioner to tighten the belt when you engage the clutch to drive the mower. You should be able to see the belt tensioner by dropping the cutting deck to its lowest point and looking underneath the mower to see which pulley actually puts the tension on the belt to make it drive. there is no need to start the mower to find which pulley needs to be adjusted, it should be obvious when you look at the belt and the pulleys.
Hope this helps you

if the belt is tight then it must be the clutch slipping the clutches cost $200-$300 so double check the belt

sounds like you smoked a belt check your blade tensioner belt, possibly an idler pulley bearing or your belt jumped off pulley, not getting right tension on belt for blade

You may need a new belt. the adjustments may need to be adjusted as well.

If you got the right oem belt,ta\han you can look at the end where the cable attachs to spring and you should see a double nut adjustment loosen nuts and adjust to put more tension on the belt when you put lever into gear for blades to turn then retighten them .