1993 Aerostar

So the other day I was driving on the highway and all of a sudden the van just stopped going 65mph and went down to 30mph. I didn't hit anything, was just driving. I would press on the gas pedal to gain speed and it wouldn't go faster than 30mph. Does anyone no what this might be? I was thinking the transmission, so I checked the fluid but it was fine. Any help would be appreciated.

Thanks
Bubba