I have come across this problem with Ford products in the past. Try cleaning off all the metal filings and rust debris that is stuck on the ABS wheel speed sensors and reluctor rings. You may find that this takes care of your complaint especially if the ABS lamp is not setting.
Herb,I did take the windstar in and the mechanic found nothing wrong at all but did try to clean things up of which exactly I am not sure but he said it is entirely safe to drive, however to me it is such a surprise when it happens that I feel if I need to suddenly brake that it may be unsafe at that time. If I were to have a complete brake job on the fronts would this help at all? Thanks again. Kurt