When you modeling, look further than the real world it self. Your shipment may need a seal, but your seal does not need to know about the shipment, or does it?
tblShipment - ID - PK
Description - Describes your shipment (You may or many not need this)
TruckNo
TrailerNo
SealID - FK
tblSeal - ID - PK
Type - (Maybe there are different seals)
Description - Describes your seal (may no be needed)
Mind you I have thrown some more fields as suggestion, not that you need them. You need to think about your model and see what makes sense for you and what does not.