some interior differences throughout the years, intercooler difference for 01's and up. rims styles changed, head lights and tail lights and third brake light changed for 01's and up. my 03 had 6 disc changer in dash, my 01 had 6 disc changer behind the seat. i heard something about 03 and up heads having 4 more threads on them or something, it was something about helping the spark plugs from poping out. 03 and up have different rear leaf springs for a higher payload. MAS is bigger for 01's and up. i think the soft tonoue covers changed too at some point. thats about all i can think of off the top of my head.
01+ have 3.73s v 99-00 3.55, 01+ also have a bigger aluminum driveshaft, as the 99/00 have a steel drive shaft that is smaller. I believe the intercoolor is a tad bigger on the 01. And as stated the 03/04 have more threads for the spark plugs.
Fun? Who needs to shift gears when you can just mash the loud pedal?