I would say it might be an issue with your employers and public liability insurance. If “hiring out /lending your drivers to other companies to drive their trucks” is accepted by your insurers as part of your business operation then it will be fine. Just ring and up and ask / tell them, if it is only occasional they might say OK, just do it 
Alternatively they might want a fortune off you 
as happened to me when we wanted to do something slightly different to our usual line of work. I changed insurers 