Fluids:
Coolant: Drain and flush your coolant and replace with either Dexcool or Zerex Extended Life (like Dexcool on steroids). Use either 50/50premix, or dilute the non-premix stuff 1:1 with distilled water. Replace your pressure cap while you're at it - get a GM cap, since most parts store caps are crap.
Automatic Transmission: Replace the fluid in your transmission. I am not a big fan of flushing as it can wash crap back into the VB. First option: drain the pan, refill, run for a few minutes to flush out the converter, then drain, replace filter, and refill again. Second option: Do the "Hoot" method. For this, you remove the cooler return line from the trans, attach a hose to the end with a hose clamp, and put the other end of the hose in a 5 gallon bucket. Drain the fluid, replace the filter, replace the pan, and fill to the "hot" line. Then, while you hold the hose in the pan, have an assistant start the engine and let it run for 30 seconds. If fluid stops coming out of the hose, that means the pan is dry. In that case, stop the engine immediately rather than going for the full 30 seconds. Re-attach the cooler line. Fill trans back to the cold line, drive around for a half hour to bring the trans to operating temp, and check fluid level while in park on a level surface with the engine running. Fill to the middle-top of the "Full Hot" hash mark area.
As for trans fluid, well, I wouldn't flush my toilet with Dex VI. There is still a persistent myth out there that Dex VI is fully synthetic - this IS NOT THE CASE. I would especially avoid non-synthetic versions if you live somewhere that gets really cold (below -30F). If you insist on using it, at least get a true synthetic version. I know it's what GM says to use, with dire warnings about what will happen if you use "non-approved" fluids. It's bull****. If you want the best, use an Allison TES-295 approved fluid. Another good, albeit expensive, choice would be Redline D6 ATF, which is their version of a 100% synthetic Dex VI. Scamsoil's version of Dex VI is probably fine as well.
Manual Transmission: Drain fluid and replace with a suitable synthetic.
Diffs: Drain fluid and replace with a good synthetic.
Transfer case: Drain fluid and replace. Don't use regular Dex VI - use whatever fully synthetic fluid you should have used in your automatic trans. Another excellent choice is Redline MTL, which won't break down and turn brown/black like the factory fill of Dex VI does.
Brakes/clutch: Suck out all the old brown fluid and fill with clean stuff. Then bleed the calipers one at a time until clean fluid comes out. Make sure to keep the MC full to prevent air from getting sucked in to your brake lines - it will take you forever to bleed it out.
Fuel filter: This should be replaced every 25k. If you have never done it, do it now and replace at 25k intervals from here on out. I bet you will find new power from your wheezing I-5 that you never knew you had lost
Check all your suspension components: Put your front end on jack stands, pull the tires, and check for looseness in the suspension components. Check the unit bearings (spit!) for play, rumble, and noise. If it seems questionable, replace it. Keep in mind that no rubber bushing in the suspension will be good after this many miles. Replace em. You can use urethane replacements which will probably last the life of the truck at the expense of slightly more transmitted noise and vibration.
Check your U-joints: Modern OE U-joints are "lubed for life"... the life of the joint that is, which may be done by the 100k mark.
Spark Plugs: Most these days have 100k change-out intervals. Don't worry about the rest of the ignition system components, injectors, etc. unless there is an issue like persistent misfires on one cylinder. These ain't like the old days of distributors and carburetors.
There's probably more, but that's all I have right now.
