[Rmowl]

Sounds like it could be the torsion spring on the deck idler pulley. More likely could be the pto belt itself. Facing the front of the mule drive check the pulley that you tighten with a 1/2" nut. Those pulleys should ideally be at about the same position if the belt is correct and in good shape. If it is a cheaper belt it may have stretched over time, had that happen. Check the spindles by removing the deck and moving the deck blades up and down to check for play in the spindle bearings just as you would move a tire and wheel top to bottom to check for wheel bearing play. Also make sure both of those mule drive pulleys are spinning freely.