In looking for transmission kits and complete transmissions, I noticed that the transmissions are listed in separate groups (2001-2003?) (2004?-2005).
What is the difference in the newer transmissions? Would it be possible to build up the allison in my 2002 and swap it into a newer vehicle at a later time?

Thanks

The newer unit has an additional solenoid that helps the cooling of the transmission. Other than that they are the same. You could swap your old into the newer truck but you would need to put the valvebody and internal wire harness into the older unit from the newer unit.