Unless it's part of an unsecured load (example: a 2x4 falling out of a stack in the bed of a pickup, or a ladder falling off a ladder rack), I doubt any court would rule the owner of the truck to be at fault. Your insurance is going to pay for the rock that cracks your windshield, not his.
Anecdote: In another life, I (briefly) drove an over-the-road truck for a living. I was on I-20 in Georgia in the wintertime and a sheet of ice half as big as the trailer I was pulling flew off the top and smacked a lady in an SUV square in the windshield. Hit her hard enough that it bent the A pillars. Thankfully she didn't crash and wasn't hurt, and just pulled over. I saw it fly off the truck in the mirror and pulled over as well. Called the police, did a report, the whole deal. Cop did not write me a ticket. Her insurance paid for her SUV and nothing went on my record.