My instincts sensed something

Paybacks are a b****. Go back to the owner of the first yard and tell him his ex-employees are working for the competition. Then ask him if he will up your price on future loads because of being short on previous loads. If he is not willing to compensate you tell him you are taking your loads to the competition. Then ask the yard manager of the second yard to ride with you to weigh at an independent scale, drop him off, weigh at the yard, then pick him up to reweigh to show how his employees are ripping him off. Since they got away with it once, they are probably doing the same thing again. If this is the case, ask both yards if they are willing to prosecute, and if not you will. Make sure you have pictures at both scales and scale receipts. Small claims court would make you the hero of all local scrappers. Make sure you contact the media. Great PR.