Also take a look at the idler tensioner pulley. If you can take the belt off, spin that pulley a bit and see if there's any amount of grab in it (It should spin very smoothly and freely). If you can feel anything grabbing hold, then replace it (They're only about $20). The bearings in those can go, I know from experience because my 98.5 Zetec with only 50k miles had the idler tensioner pulley go, and it made the belt squeal all day. Basically it's making the belt slip because the belt's grabbing on that pulley.
If you feel like changing it out, then I would suggest getting an odd-end ratchet with a 1/4 inch drive.
