Rear of the car feels unstable at highway speeds.

Hello, definitely am agreeing about the tire replacement causing your new concern. I would suggest checking tire air pressure first. I would also check having the alignment checked again and make sure all angles are correct on a four wheel alignment, not just a thrust angle alignment. Something is definitely not right here, but it's impossible to figure out until you get somebody with an interest in helping you out checking everything along the way. If it's not the dealership, find an independent shop near you that can help. Use this link to find some shops near you that can help with your concern: www.repairpal.com/repair-shops