Why won't my 2003 windstar transmission engage into any gear except park?

It is most likely a front pump failure; its very common with these vans. You can check by removing upper transmission cooler line from the the radiator and starting engine. Place a pan underneath to catch fluid. You should get a minimum of one quart per minute flow. If you get no flow you get no go.

Also have your engine codes checked. Failure of the PCM will cause this problem also. Do not have your trans rebuilt until you check this. Water can get on the PCM causing all kinds of disruptions including trans failure.