Answer
Your reverse sun gear weld to the drum has more than likely broken. this was a common failure on ax4s transaxles of around the 1999 to 2000 model years of windstars and tauruses. the symptom you describe is what typically happens when this specific failure occurs. look for a round grey coloured electrical connector situated on the top side of the transaxle, near the air filter housing. if you disconnect it, you might be able to drive it to the nearest repair shop. what this will do is put it into fmem mode. to repair this problem will require transaxle removal and disassembly, and depending on the extent of damaged parts found, maybe even replacement with a rebuilt unit. hope this helps.