BILL, PATRICK - You don't mention which mower you have on your 128, may have been in an earlier post but I didn't check. If you have a 38, 42, or 48 inch deck the mule drive should have an adjustment stud/bolt on the idler pulley without the spring. You should adjust that so the two pulleys are about the same distance from the front of the mule drive. With the correct belt they should be in line right under the PTO clutch pulley.
If you have a 44" deck, that mule drive should have the release lever which really doesn't allow adjustment of the alignment for the two idler pulleys.
As long as I've always used IH/CC belts I've never had a problem throwing the mule drive belt off. Not all other belts like all the twisting & bending the mule drive belts have to do. Guess I've been lucky, but I remember KRAIG saying he had a problem throwing the mule drive belt once and he "Reversed" it, problem solved. You might try that too. V-belts have a "memory" and don't like to run backwards, just like tires.