Well, I'm not familiar with the 250s personally, but it has the 5.4L engine so I believe you will have the 4R75W transmission which is just the newer version of my 4R70W. I'll be honest though, I don't know much about transmissions, but these are some things I've read about and had seen others experience first hand. My extent of transmission work is only performing a J-mod (drilling holes and removing springs from it).
Before you read on, eliminate all of the usual suspects - check the fluid. It should be properly filled and have a bright red color. If it is low or discolored and smells burnt, this may be your problem. If it got to this point, however, it would probably call for a transmission rebuild anyway
Moving on, I know there was a problem with plastic thrust bearings breaking apart, but I believe it was eliminated in 2003 to a revised one. If yours is plastic and it did break, it would cause your symptoms. This requires the replacement of the TC and a full transmission rebuild (all the pieces get into the clutches, valve body, and cooler).
Some better news may be a solenoid has failed. These are more easily fixed with dropping the pan and valve body and replacing. The MLPS sensor may also be to blame in some cases. The MLPS or incorrectly called the "neutral safety switch" is used with the PCM to command different gears and if it is not working properly, the transmission may not shift at all.