Another very slim possibility is both bulbs have low beam out. Check at the bulb for power to the low beam side of the bulb.

I had both bulbs go bad on low beam one time and about drove me nuts till I pulled one out and checked the power at the bulb connection.

Wound up replacing both bulbs to get low and high beam lights back.

Always start first at the light not working to make sure you have power to it is how I work an electrical glitch. If you have power to both low and high beam sides of the connector, then the bulb may save you a lot of work.

If no power to the connector, then work your way back to the connector on the fire wall on the drivers side to see if there is power there. Or go directly to check the dimmer switch.

Another easy spot to check is the dimmer switch. It is possible the dimmer switch has burnt the contact inside for the low beam side. You'll have to have someone inside the truck to activate the dimmer switch to make it a bit easier.