1992 cougar/tranny won't shift

I have a 1992 mercury cougar with a 3.8 V6 and a aod tranny. One day the tranny all of a sudden would not shift from first to second until the rpm's were up over 4000. It will do this in every gear, and the odd time will not shift into OD. Also sometimes when stopping from highway speeds and then accelerating from a stop, it feels like it never downshifted and is stuck in third gear. The fluid is still real clean, no water or burning. Should the tranny be replaced or is it something simple? Has anybody had the same problem?