Gi8u2

My 1991 F-150 2WD A/T, 5.0 V-8 recently started shifting from 1st, then straight to overdrive. It does this once its warmed up. Fluids are not low and not old. Truck only has 107K miles. It started doing this while on a trip, so as a workaround, I took it out of OD and shifted down to '2' on the column and essentially, I became the automatic transmission. It has not been abused and only recently was used for towing. Any ideas what I'm in for?

Steve