I would have the rear in gear oil changed, That dirty gear oil is whats keeping your rear wheel bearings lubed, Radiator antifreeze needs flushed, It will eventually get old and you'll end up with a heater core that won't heat and a radiator that wont keep your engine cool, Transmission fluid get metal shavings in it as it gets older eventually wearing other parts out. I think your a pretty lucky guy that you have'nt had some major failures to your truck so far. The cost of not doing maintenance can get pretty expensive quick