As long as the state you are living/driving in allows for a CCW then its legal. However, if the company you work for has some kind of rule you might risk getting fired if somebody found out. That said I would carry one anyway the odds of getting caught when you are alone in a truck 99% of the time are pretty small. Besides if you ever had to use it to defend yourself losing your job would be the least of your worries. I would rather survive and find a new job than get mugged.
Check out what happened to this poor truck driver.
http://www.metacafe.com/watch/yt-_Mw...eginald_denny/