Final drive ratio is different between the 3.3 and 3.8. PCM won't be happy.

 

Transfer Gear Swap
You can make it work reasonably by playing w/ the transfer gears.

That said, I know there are some electronic control changes. Not sure if you may have an issue with that end of things.

 

The final drive ratio is different, but the PCM won't care. The pinion factor is stored in the PCM/TCM. Your speedometer will be slightly off, or you can have the PCM/TCM recalibrated, or you can swap the differential ring gear, transfer shaft(pinion), and transfer gearset from the old transmission to the new one.

 

As long as '03 is FWD, it can be swapped with some additional work (transfer gear swap, perhaps TRS / solenoid / speed sensors). The box will fit, transfer gears and the rest can be swapped with 41te in the van.