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Do you have front running lamps or are they also out?
If both front and rear are out, it's probably a fuse, relay, switch or similar. If the fronts work but the rears don't, it may very well be a wiring harness or connector problem.
if the front lights work start with the rear bulbs, they are a dual filament bulb, unusual for both to go out at the same time but it can happen. If it is not the bulbs I would suspect a harness connector issue, ie. corrosion where the rear harness plugs into the harness to the front.
look over the connectors closely - not uncommon for bulbs to not make good contact w/ the connectors. These trucks are getting just old enough that this kind of weird stuff starts happening. The sockets are cheap ~ 20 bucks.
so tried swapping out bulbs that want the problem then when i put the old ones back in they decided to start working. and then now they are back to not working again. WTF???