It's absurd to believe they were fired from a pickup truck since the pressure alone would blow through the bed. Note that multiple rounds were fired and accurate adjustments made IOT gain effect on target. These weapons were well/professionally layed in advance of the assault. As a Marine Gunner, Infantry Weapons Officer, Retired, with 34 years of service, even I am impressed with the rapid accurate fire of this mortar crew.
No Infantry Officer would ever make a foolish statement like Petraeus unless he purposefully intended to mislead the congress. His statement was all about protecting something or someone!
Thank you for your knowledgeable reply Gunner.