This sounds like a main power wire that has a poor connection. If all the lights are powered through the main headlight swith, which I think they are, then I would start there first. Maybe the switch itself is going bad. With the truck off, turn on your lights and watch to see if they dim, try wiggling the shaft of the switch to get them to flicker. Hopefully it is just a loose connection.

52 Chevy 3100 5 Window, LS Swap, 4spd Auto, Vintage Air AC / Heat, CPP Power Front Disc Brake Conversion with bearing upgrade, Power Steering Conversion, LED Tail Lights