A lot of this depends on how you use the truck. If all you're doing is trips to Home Depot and parades, then you can radically decrease the frequency of things like hub service. But if you're in the mud and water, or crawling rocks, that changes things. Any time you're in mud or water that's hub deep (or more) you should pull the drums and inspect for contaminated bearings. Same goes for the gear oil. Severe off-roading also calls for more frequent greasings and dogbone inspections.
My personal regimen, with a truck seeing very limited off-road use and less than a thousand miles a year is as follows:
Complete hub service-pulling all the drums off and inspecting bearings and seals and wheel cylinders, etc. Every 2-3 years. Honestly, this schedule has been driven more than anything else by the short life of the front axle boots.
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Oil change and fuel filters once a year. Squirt-truck has been having his oil analyzed in his M54A2 and it is lasting several years and he is putting more miles on his truck than I am so my schedule may be overdoing it. YMMV.
Zerk greasing twice a year. This is driven more than anything else by the fact that the truck has 8 u-joints spinning at all times and I don't want one to fail 200 miles from home.
Coolant and brake system flush every 2-3 years.
Once a year, I take a couple hours to very slowly go over the entire truck. All the things that can go wrong with older vehicles-brittle plastic fuel lines, rotting radiator and heater hoses, pinion seal leaks, loose driveshaft bolts, broken parking brake springs (a common issue), parking brake shoes wearing out, drive belts with cracks, radiator leaks, frayed wiring, copper air lines that rub against items, etc. You literally want to put eyeballs on every component of the vehicle. Crawl under it and start at the back and work your way u to the front. Stick your head under the dash.
With what I've described above, I've been able to keep my truck(s) in a condition that has allowed them to be mission ready more or less nonstop. There are unexpected failures, of course: When the injection pump seals wore out and allowed it to leak diesel into the crankcase. Or an axle seal failure. These are things that no amount of inspections can catch.