Who is driving FLAG truck?

Sometimes it's a human and sometimes it's on autopilot. They explained it in the one episode with RC3.

Terri "The Tiger" Calley (Yvonne McCord) in "Knight Moves. so I'd like to pretend she did regularly.

Maybe the Highwayman or Jetto?

There were apparently many off-screen employees of FLAG, who were not essential to the story. But yeah, by just always showing only Michael, Devon, and Bonnie/April every episode, it gives the impression FLAG is not just a small business, but a TINY business! I recall only one occasion, I think in one of the Goliath episodes, where they showed a FLAG receptionist answering the phone right before she's kidnapped.

Just a little joke here, maybe it was Jerry Reed ("Snowman") from Smokey and the Bandit who drove the truck! Wouldn't that have been a funny revelation?

KATT...IT'S KITT & KATT.

I think its supposed to be the truck driver from the movie Duel. However he is now rehabilitated, after jumping out of the truck just prior to it going down the cliff. Thats why they purposely never showed him in the series as paying homage to Duel