Sounds like the wires are misconnected on the headlights,ie,the ground is on the wrong headlight bulb terminal.To check, pull of the bulb connector and hold it with the business end facing you like an inverted U, one terminal on the top,one on each side.the left side is hi beam,top is low beam,right is ground.Then run a separate ground wire back to the battery negative for all lights on the truck,relying on the chassis for electrial continuity is a poor idea,car makers finally wised up on that 15 years ago.
And check the dimmer like mentioned above.
