I am with Gord. It can be a loose or half broken wire. Making and breaking due to vibration/shaking while driving. It can be a connector with corroded contacts and the vibration, again, causes intermittency, You would want to see if it does it sitting still with motor off. Then start motor and check. Then shake truck and it's wires.
If you think it's the dimmer switch, there are three terminals. Battery (B) terminal and the other two terminals are Lo beam and Hi beam. Does not make any difference which is which for those two. Just take off either the lo or hi wire off it's terminal and put it on the battery terminal. (Two on one terminal). Turn on lights and take it for a ride. Then put the wire back on it's location and try the other onto the battery terminal. Drive it again. This will bypass the switch and tell you good info. Logic will then solve it. Report results.
It's OK can to consider a short, but don't focus on that alone. May not be a short. I would also check those terminal blocks on the inner fender. I always recommend getting rid of those and splicing wires. (Solder wrap and heat shrink sleeving)
Do not dwell on the "interrupter" or the headlight switch.
I believe each headlight has it's own separate ground wire attached to the immediate chassis. So if both headlights are going out, it's not that particular ground. One will not affect the other. The only other grounds involved are the large frame to battery, body to battery, motor to battery ground combination you have.
