Les Schwab stores regularly deal with larger tires and have a strong customer service ethic so it is unlikely that the issue is lack of skill or lack of caring about doing a good job, particularly if they have invested time and energy trying on multiple occasions without receiving additional compensation. They want the job done right so they can move on to the next one and so you can recommend them to your friends and neighbors.
My suspicion is an out of round tire or out of round or bent wheel. Be sure both run true.
Two things you can try after you have eliminated bad tires or bad wheels as the cause of your issue are "road force" balancing and static balancing. The first typically works better for smaller, lighter tires but sometimes is the answer for offroad tires. The second method is largely a lost art because spin balancing is faster and less effort, but sometimes it is the best alternative for big, heavy tires if the tire man knows what he is doing.