OT Mechanics - new tire balance problems

ill try to be brief. i purchased 4 new Continental 275/55/20 tires and had them installed on my 2015 GMC 1/2 ton (2wd, no mods, replacing with same exact tires that were on truck when i rolled off the lot). my first time at interstate speeds, my center console is rattling. bring truck back, shop removes and rebalances all 4. back to interstate, same problem.

i bring truck back a second time and the shop replaces 2 of the tires (rear). according to the shop owner, rattling in the center console indicates rear out of balance. back on the interstate, there is rattling, but it seems to be on the floorboard.

for a second opinion, i go to a shop with a spinner that can spin the front wheels of the truck while on the truck. im there when the tech spins them and there is a noticeable shake, on both front tires, left much worse. Back off to the original shop and he sends me to an Allied Discount Tire & Brake for a balance job, at his cost. After balancing, the Allied manager gets the truck to 75 on interstate and comes back and reports no shake. My original shop rep and i then both get in and get truck to 90 mph for about 5 miles. there is a very slight shimmy but not near as bad. At this point i feel like i was hyper-vigilant and the shake was nothing out of the ordinary at high speed. i tell him its good, drop him off at original shop and am on my way

the Truck has now started to shake again in the center console at speeds of 75 mph and up, though not quite as bad as the original shake. Im looking for any diagnosis from anyone here with experience. is it possible the 2 front tires, which are still original new tires, are out-of-round? Should i just take it to the best shop i can find for a 3rd opinion balance job? any opinions are appreciated.

thanks