The web link below lists 13 transmission assembly choices for the 2003 Dodge RAM 1500 Pickup and only 8 for the 2004 Dodge RAM 1500 Pickup so there must be some incompatibility. I suspect you will have to get more specific than "a transmission" to be sure, like identify the model. The web page has a telephone number that may be able to provide more info.
http://www.autopartsfair.com/dodge-used_transmissions/ram_1500_pickup-2004-catalog.html?campaign=theautochannel