KMC91fox

I have a 91 5.0 with the aod tranny today coming home it started shifting though 1st-2nd-drive-overdrive real fast then slips in OD , if i leave it in drive it still shifts 1-2-3 real fast but doesnt slip i tried leaving in it 1st and uit was fine, will burn up the tires but the OD problem puts me at a loss. I have only owned the car about 3 months and i love it but i also dont know alot about it , i was a chevy man but after this car i am hooked on the 5.0 mustangs. can anyone help???

2002blkgt

maybe low on auto trans. fluid. might want to check just in case. otherwise don't know much about auto.