I had a drum go in my 98 Tour SE ATX... same symptoms you're describing. One day it's good to go, the next I'm driving and it just slips and slips and then nothing. I got a rebuild done on mine to the tune of about $1200 CAD... but it was a cash deal. I'd get it looked at though, if you're not to inclined with a wrench you'll probably want to avoid trying a rebuild yourself. It can get ugly in there.
----------
