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.