1992 tranny problem

I have a 92 bronco XLT with the 5.8L V8. Whenever I try to accelerate, my truck slowly starts going and wont switch gears. I have to tap the gas pedal a couple times before it switches gears. I checked the tranny fluid when the truck was running and it was perfect. Tranny was flushed in the Fall of 09. Any help is much appreciated.

Replace the VSS (variable speed sensor).