question regarding union workers picketing at a nonunion store

I am a worker for a company that has unionized and non-unionized stores. I am part of a non-union store and at the moment our unionized stores are on strike. I’m curious as to the legality of the unionized workers picketing at stores that are not unionized in order to block our trucks from being able to deliver product to us. The way the system is set up is that delivery drivers enter through the front of the store to access and unlock the back delivery door in order to deliver product, and because the delivery drivers are unionized as soon as they see the “picket line” they leave. Is the picketing of stores that are not unionized something that is actually allowed legally? I’m mostly asking because the “picket line” I am referring to is only a few people in a single vehicle who are following the delivery trucks and getting ahead of them in order to stand in front of the entrance right as the truck gets there, as soon as the truck driver turns around to leave they get back in their vehicle to head to the next store on the delivery route.

From what I’ve been able to gather from some searching and some reading I’ve done it could possibly be illegal but maybe not, it got confusing and it’s late and I’m tired. I’ll also be the first to admit that sometimes I misinterpret things. Thank you in advance to anyone who is able to give me information. If it is something that is not legal I really want to let the people who are doing it know since the company we work for would not hesitate at all to terminate the employment of the 3 people involved. And if it is legal then okay I guess, just kind of hoping our store doesn’t get temporarily shut down if we end up running out of product, because unlike the union workers who get stipends from the union while striking, we won’t get paid if that happens.