Tires, air dam and synthetic fluids all sound spot-on.
I'm no diesel tuner, but my reading has suggested most diesels have severely power- and economy-compromised tunes, because there's a very large tradeoff with emissions (think back to the VW TDi scandal). Tuners (as I understand) mainly adjust injector timing to get these gains. It's not something you can do easily without a dyno, but these trucks are old enough that the changes to be made are extremely well documented and you should see large improvements with a canned tune.