So, the spindle is OK, and you have a slipping belt for some other reason. Is this a two-belt setup, with one belt for spindle drive and another belt for wheel drive? If it is, we need to find out which belt is slipping.
1. Does the deck move the same amount vertically as it did back when the belt didn't squeal? You said the movement of the deck is jerky: it hangs and then drops. It sounds as if something may be sticking there, and perhaps the deck isn't moving to where it has to go.
2. Look at the bottom of both of the belts, and at the bottom of both grooves in the pulley, and see if there are signs that one of the belts is touching the bottom of a groove.
