The biggest difference you will find in model years will be options. The 02s had the light grey cladding and the older interior that was used in the pickup & tahoe/suburban line from 1999-2002. Then the new interior came out which all 03+ Avalanches would have. The 03+ also has a darker grey cladding. GM started taking away features on us for some reason so I will post a link at the end to another forum that I belong to that has a great resource section. There you can go into each year and see what changed from the previous year. Things like the under hood light and cabin filter were left out of the 03 when it came out. Then the dual visors were taken out of the 04s and one of the side bed lights. There are other things but those are the things that were dropped from year to year. Oh and nothing was really added to make up for the loss. In my opinion the 03 was the best year as far as what came with it and is what I have. How ever if you find a great 04 go for it. The things taken away can easily be added if you want. The bed light being the most difficult but it has been done by many people.
I know of many 02 owners that wish they had an 03+ because there are more aftermarket items available for it. The biggest one is the backup camera that has a small 3.5" LCD that replaces the cubby hole to the right of the HVAC controls. I love mine and it isn't available for the 02.
Here is that link
http://www.chevyavalanchefanclub.com...ces/index.html